Is it possible to create or edit a hair / hats, glasses, mustache weapons or maps of Ultimate Mayhem Worms?

Sorry for my English. I come from Poland.

Unfortunately you can't create your own costume items for Worms Ultimate Mayhem. Sorry about that.

Or maps, or weapons (outside of the in-game team weapon editor).

It is possible to edit and make maps! (However, you must replace existing maps) http://forum.team17.co.uk/showthread.php?t=49562

Creating new items works the same way, you need to replace them. Tweaking weapons should also work somehow, as long as the tweak files are not stored in one of those nasty .xom packages :(