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There are a bunch of different ways to do this. The problem is none of them are great. They leave a lot to be desired.
The term is "short urls" which can be done with "mod rewrite" on an apache server. Try searching those terms there are 100's and 100's of posts on the matter. It will take days to read them all and understand.
Here is a module that does short url's
Xoops Hack. It does a bunch of other stuff as well. It is still a rc version. It might be worth testing on a non production site to see if you can get it working first. Test test test...
The next XOOPS version (xoops 3) for lack of a better word should have this feature right out of the box. I have no idea when this release will be. It will be awhile so do not hold your breath.
Good luck...