Hello Tor André Solbakken
Thank you for posting your query on Microsoft Q&A. As per current design we don't have a way to do a user writeback from Azure AD to On-Prem AD.
However our product group is aware about the request, but have no timelines on when this feature would be available or would be on roadmap.
You could upvote and share your comments with business requirement here as our team has visibility to the feedback or post a new idea.
But we have a group write back option for Cloud Groups. Group writeback allows you to write cloud groups back to your on-premises Active Directory instance by using Azure Active Directory (Azure AD) Connect sync. You can use this feature to manage groups in the cloud, while controlling access to on-premises applications and resources.
As this is in preview kindly validate the limitations.
Please do let me know if you have any further queries for same.
Please "Accept the answer" (Yes/No), and share your feedback if the suggestion answers your query. This will help us and others in the community as well.