essentially yes. Just add the foam you want, shape it, then add a skim coat of bondo, sand that to a perfect surface, then you can pull a mould off of it.
One little tip for cheapskates: Instead of bondo use drywall compound. This can be shaped easier, and is MUCH cheaper. Then take a primer (house latex or similar) like Klitz and spray prime it. Then you can take very fine sandpaper and wetsand it to perfection.
Add some release agent (PVA or Wax... some have luck with either... DO NOT USE BOTH!) Spray on your gel coat (generally for the mould in whatever color was cheapest at the time

) and then lay up as many layers of chopped/mat glass as you feel nessesary. Then you will have a mould that you will be able to pull pretty much infinate parts from... well, until you break it
HTH
Matt