Nothematic

Same as replacing vehicles, just you go to component peds instead of vehicles in the spark 

I think it may be possible, but it would require extensive editing of the following files;
 
handling.dat
carcols.dat
vehicles.ide
cargrp.dat
 
Like I said, it may be possible through the modifying of those files, take note of the last one as it controls which pedestrians drive which vehicle. There is a file somewhere that decides which cars spawn and where but I'm having issues finding it.