Uluru who is known for creating the GTA3 and Vice City Garage Editors has already come up with one for GTA San Andreas! The first version only allows you to edit the cars in Los Santos, but Im sure he will be able to create better versions as time goes by.
This first version only includes the garages in Los Santos, I havn't unlocked the other cities yet so I can't really test the memory addresses that the cars are stored at.
This tool lets you change what cars are stored in your garages, the colour of them and whether they are damage proof. It also lets you lock them in there so everytime you open the garage the cars will be waiting for you.
Just remember that if you add a car into a garage that had no cars in it before, will cause your game to freeze.
Click here to download.