@Guido Gibens Thank you for reaching out to us.
As far i am aware Azure AD Domain Services does not support custom schema extensions. Extending schema is not a permitted operation on the AAD Domain Services instance.
However i will check in my lab whether your ask can be achieved or not. Please give me coupe of days time to check and revert.
Also do you have any specific reason why you want division, employeetype attributes synced from AAD to AADDS ?
Reference : Attribute mapping for user accounts from Azure AD to Azure AD DS
https://learn.microsoft.com/en-us/azure/active-directory-domain-services/synchronization#:~:text=specific%20attributes%20for%20user%20objects%20in%20Azure%20AD%20are%20synchronized%20to%20corresponding%20attributes%20in%20Azure%20AD%20DS.