If you didn't understand from my last post, I'll explain. MapVote mutator (this window on screenshot with maps) generates the list of maps from either of two ways:
a). Takes it from Maplist in DeusEx.ini <--- you're limited to 31 map.
b). Generates it from Maps/ folder, independently to DeusEx.ini <-- you're not limited at all.
So, when using way b) you may just put .dx file into Maps/ and it will automatically appear in the list of maps in the mutator.
Upd:GuardianDevil wrote:but won't I have all the single player maps as well?
No, there is a variable like bUseSingleplayermaps which is False by default.