How to add some schemaextensions to Active Directory:

1. Find the ADDC with the Schemamaster FSMO:
root@addc-01:~# samba-tool fsmo show
SchemaMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net
InfrastructureMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net
RidAllocationMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net
PdcEmulationMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net
DomainNamingMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net
DomainDnsZonesMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net
ForestDnsZonesMasterRole owner: CN=NTDS Settings,CN=ADDC-01,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=example,DC=net

schemaextensions can only be managed on the ADDC with the SchemaMasterRole.

2. Check if the Replication is working between alle ADDCs
root@addc-01:~# samba-tool drs replicate addc-01 addc-02 dc=example,dc=net
Replicate from addc-02 to addc-01 was successful.
root@addc-01:~# samba-tool drs replicate addc-02 addc-01 dc=example,dc=net
Replicate from addc-01 to addc-02 was successful.

3. If you don't have a own ODI for your attributes get one from http://pen.iana.org/pen/PenApplication.page
It will take a few days until you get your free ODI. 
All your attributes must have a unique ODI, it schould look like: 1.3.1.5.1.4.1.<your-ODI>.<number> . To make your attributes unique you should use as prefix for all your attributes.

4. You need two ldif-Files for adding the extension, one for the attributes and one for adding the attributes to the Objectclass.
File for attributes:
---------------------------
# File attrib.ldif
# => .name
# = "String(Unicode)",
dn: CN=StkaTextAttribute,CN=Schema,CN=Configuration,DC=example,DC=net
changetype: add
objectClass: attributeSchema
attributeSyntax: 2.5.5.12
oMSyntax: 64
attributeID: 1.3.1.5.1.4.1.987654.1
lDAPDisplayName: StkaTextAttribute
adminDisplayName: StkaTextAttribute

# => .name
# ="Boolean",
dn: CN=StkaBoolAttribute,CN=Schema,CN=Configuration,DC=example,DC=net
changetype: add
objectClass: attributeSchema
attributeSyntax: 2.5.5.8
oMSyntax: 1
attributeID: 1.3.1.5.1.4.1.987654.2
lDAPDisplayName: StkaBoolAttribute
adminDisplayName: StkaBoolAttribute
---------------------------
To find the attributeSyntax you need downlaod the sources from Samba4, open file source4/dsdb/schema/schema_syntax.c 
and search for: "static const struct dsdb_syntax dsdb_syntaxes". There you will find all possible attributeSyntax.

So replace "987654" with your ODI-Number, choose your attributeSyntax and the attributename and create your attribute-ldif.

5. Create your class.ldif to add the attribute you created.
---------------------------
# File class.ldif
dn: CN=User,CN=Schema,CN=Configuration,DC=example,DC=net
changetype: modify
add: mayContain
mayContain: StkaTextAttribute
mayContain: StkaBoolAttribute

dn:
changetype: modify
add: schemaUpdateNow
schemaUpdateNow: 1
---------------------------
This ldif will add new attributes to all your users.

6. Add the attribute to your Active Directory:
--------------
root@addc-01:~# ldbmodify -H /var/lib/samba/private/sam.ldb --option="dsdb:schema update allowed=yes" --verbose < attr.ldif
Modified CN=StkaTextAttribute,CN=Schema,CN=Configuration,DC=example,DC=net
Modified CN=StkaBoolAttribute,CN=Schema,CN=Configuration,DC=example,DC=net
Modified 2 records successfully
--------------

7. Add the attributes to your Objectclass:
--------------
root@addc-01:~# ldbmodify -H /var/lib/samba/private/sam.ldb --option="dsdb:schema update allowed=yes" --verbose < class.ldif
Modified CN=User,CN=Schema,CN=Configuration,DC=example,DC=net
Modified 
Modified 2 records successfully
--------------

8. recheck replication:
--------------
root@addc-01:~# samba-tool drs replicate addc-01 addc-02 dc=example,dc=net
Replicate from addc-02 to addc-01 was successful.
root@addc-01:~# samba-tool drs replicate addc-02 addc-01 dc=example,dc=net
Replicate from addc-01 to addc-02 was successful.
--------------

9. Create a ldif to change an existing user:
--------------
dn: CN=u4,cn=users,DC=example,DC=net
changetype: modify
add: StkaTextAttribute
StkaTextAttribute: What is the answer to all questions
-
add: StkaBoolAttribute
StkaBoolAttribute: TRUE
--------------

10. Add the attributes to the user:
--------------
root@addc-01:~# ldbmodify -H /var/lib/samba/private/sam.ldb user.ldif
Modified 1 records successfully
--------------

11. List all attributes of the user:
--------------
root@addc-01:~# ldbsearch --url=/var/lib/samba/private/sam.ldb cn=u4
# record 1
dn: CN=u4,CN=Users,DC=example,DC=net
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user
cn: u4
instanceType: 4
whenCreated: 20161005151014.0Z
uSNCreated: 3830
name: u4
objectGUID: 4dd53b1f-8378-442a-b2c2-3cf641856ffd
badPwdCount: 0
codePage: 0
countryCode: 0
badPasswordTime: 0
lastLogoff: 0
lastLogon: 0
primaryGroupID: 513
objectSid: S-1-5-21-3254471717-23904020-1408941044-1106
accountExpires: 9223372036854775807
logonCount: 0
sAMAccountName: u4
sAMAccountType: 805306368
userPrincipalName: u4@example.net
objectCategory: CN=Person,CN=Schema,CN=Configuration,DC=example,DC=net
pwdLastSet: 131201538144805200
userAccountControl: 512
StkaTextAttribute: What is the answer to all questions
StkaBoolAttribute: TRUE
whenChanged: 20161030182528.0Z
uSNChanged: 3996
distinguishedName: CN=u4,CN=Users,DC=example,DC=net
--------------
Now you can add your attributes to all new and all existing users. You can also use ADUC from the RSAT to add the attributes. To do so, activate the extended options 
then uses the "attribute-editor" to edit your attributes