As far as I know, you can't really add anything to the Great Stuff foam since it's already mixed when it comes out. You will notice that once you start to cast your objects, they will shrink some when you take them out as they cure.

There has been very mixed results using the Great Stuff foam since it does need air to cure properly. You will need take your time with it and expect not to have perfect copies every time.

Good luck and let use know how it goes.