There's a way to convert 3d meshes (ie, from blender) to .map brushes. I haven't tried it yet, but it basically goes like this:
Create your 3d mesh, export it as a .ASE file (there's supposedly a .ase exporter for blender on doom3world.org) and put it in the quake3 models directory: quake3/baseq3/models. Open up radiant, quark, or some other q3 supporting level editor and place the new model as a misc_model:
"classname" "misc_model"
"model" "c:\quake3\baseq3\models\yourmodel.ase"
"spawnflags" "6"
Seal the map up with walls and add an info_player_start. Compile it with q3map2 (no need to do vising or light). Run q3map2 again with -convert -format map <mapname>. This should give you a q3 format .map file with the mesh broken up as brushes. From there you should just be able to open it and resave it as a quake2 map file (that may not even be necessary).
Sources:
http://www.quakesrc.org/forums/viewtopic.php?t=5148&start=2http://www.quakesrc.org/forums/viewtopic.php?t=4810&start=16