Hello, and welcome!
The current user portal does not and will not enable other customization than adding a theme. There is a rework coming up that includes an overhaul of the login API (that might include sending information about the app category from the catalog), and the portal UI.
For an overview of the ongoing work: [META] Tracking issue for user management, sessions and SSO · Issue #2239 · YunoHost/issues · GitHub
There is and will be no timeline on the completion of this major work, but contributions are welcome. 
Depending on your use case, if you do not need the portal to be dynamic (regarding the users permissions) right away, you might consider creating your own startpage (Flame, Homer, …).