Hey @amy.blais, thanks for the quick answer 
The thing is (the title is misleading probably), I can’t rename the users on the server, since I am not the admin.
The question was more about how I could change the name displayed for me. I didn’t find any contact management whatsoever, there is only one global user list.
The functionality I’m talking about would require to save a list of contacts for every user, with settings you can change for each contact. (basically the roster one has on xmpp servers).
I’m assuming this is doable with a custom client, but then I’d have to change the names on all clients, or implement client sync.
Example:
Other user has the name I-have-a-nickname-with-text-you-dont-want-to-have-20-times-on-your-screen-while-chatting-to-me which I would like to change to Tom, and talk to him like this, no matter how he sets his name.