Board Thread:Support Requests - Getting Technical/@comment-4748628-20140318152628/@comment-168424-20140318222757

It is possible, but unlikely, since it would probably require major PHP surgery of the wiki engine and diverge your wiki from the common implementation dramatically.

You should start by asking Wikia if they would allow something like this via Special:Contact/general. There is about a 99% chance they'll say no, but you can still ask.

Addressing your idea, it doesn't seem like a really necessary solution to have a separate namespace for a separate set of categories. I would just make it wiki policy to have users put categories specific to their uses only in Category:User categories or something like that. Where are you worried these user-specific categories will show up that you need to make a complete separate namespace for them?