As was said above, just alittle bit adding on:
If the game uses other engine than Q3 it is most likely that you will have to start map from scratch, or if you are a good programmer you might write a script, which will convert the map for other game into a desired *.map for tremulous, it will save you all the brushwork, but as for textures, shaders and entities, you will have to rework them.
With Q3 engine based games its a bit easier, you just need to remove incompatable entities and shaders from the map (and script files as well) and replace them with tremulous analogues.
Personally I have converted the CTF-Face map for WolfET, and it took about 48 hours to port all the brushwork, texturing is another story, and all due to the fact that UT engine allows you to use substract brushes, whether Q3 engine only allows ADD brushes.
