You don't have to start from scratch, though.
There are example codes for creating meshes in the forum and in resources, from fogman and ventilator as far as I know.
And, there is the dll from TechMuc to save a mesh as an mdl.
When you got your mesh as mdl, then you have the common collision system.

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt