Cliff, I did a yahoo search Sims 2 file format and found the following site
http://www.TNLC.com/eep/sims/sims2html for an insight into the file structure.
If there is no animation to what you want to add like a vehicle or house the format is .mdl or mdl2. You need to look at the game for a file drop down list and a file import entry. from ac3d things like ,obj, wav(wavefront) .stl, 3ds,
3dsmax would be candidates to look for. Windows explorer might be able to display the files in the game where you can find a folder with the file extension in the above link pertaining to what you want to add. Then place the new file in the folder of similar files.
If you are attempting to enter an avitar or animated model things are much more difficult. It appears that there need to be several files with associations such as a .anim file for animation and a .behv file for behavoir and possibly some others in the 35 listed file extensions. This would seem to require a third party program dedicated to the sims2 and sims3 interface. The model would be constructed in ac3d due to the easy more intuitive interface than similar programs then exported as an .obj file into the third party program.
Often the site which owns the game will have a downloads or resources
list where you can get something used for development of the game additions
wizzardworks