it is possible but also fairly messy depending on the object you want to have modelled, and requires quite a lot additional work after the actual conversion.
quark has some model editing features; HeadThump once sent me a program called 3dexploration which could load maps and export them to a format readable by modelling apps; and a related (albeit for q3) was posted somewhere recently.
although, if you just need to bring down polycount, there are better methods you should try first, e.g. turning objects into func_walls, moving them 1 unit away from the adjacent surfaces, and upscaling textures.