Hi @Robin1988 ,
i'm afraid you can't do that. To my (may be limited) knowledge everything in portals is around contacts. You have the permission system, the local password stored there, the external identites are tied to the contact and in portal pages itself you get the user-objects, which is in fact the logged in contact.
I think, you have to customize the contact entity when you have extra infotmation. May be, you could use a lookup on your entity to the contact and allow the editing of that data in a contact scope. (as it will be a 1-1 relation you could also put the lookup on the contact).
So, i think not possible due to the lots of places where the contact is wired to.
Hope this helps, even if it doesn't help,
Christian
