I have recipes that are fixed and I add the glitter to the raw plastic. I know there has been some discussion about glitter colors that bleed. Purple seems to be a popular one, but I haven't had any issues in adding it to the raw plastic nor in reheats.
If you are concerned about the glitter thing, you can add it when the plastic is cooked.
Keep in mind that the glitters you order from CCM are all compatible with microwaving. Glitters gotten elsewhere are probably ok too, but if you happen to land on a metal-based glitter and toast it up in the nuke machine you are going to have fireworks. Play it safe and get the glitters here until you learn which ones you can use safely. Again, purple is a problem color in this regard and I think I heard of a pink thats got the metal base too.
Its probably a good idea to add anything to re-melts or old plastic until after it has been melted just to cut down on the liklihood that anything will go south on you.