actually you can use any 3d-program that is capable of making low- or midpoly models as long as it can export in a format readable by blender (f.x. 3ds or lwo) or any other program capable of writing ac-format-files, a bit of cleanup of might propably be necessary though.
Here:
viewtopic.php?f=3&t=79&start=10 is also a link to a tutorial (it's made for torcs but speed-dreams uses the same files) which has many pointers for importing (f.x. handling of transparent areas, offset etc.) cars for use ingame.