Go to the main menu and go to console ( by pressing ~ on your keyboard or the button on top of the tab button)

From there type :
/mod_map or /mod_map_data ,I can't remember.This varies with different versions of course.

Then use your own 3d modelling programs and textures to modify the maps as you wish.I recommend polygon programming for objects and more complex models.You also might need a file convertor for the textures.

Ugh..k,I will get more valuable information than this soon.Hope what I wrote already gives you an idea.

Thank you