Stocking numbers would be different since you already have a crappie population, than what they would be in a pond devoid of fish. The big unknown is how successful the crappie spawn was this year; if it was relatively unsuccessful, and there are only a few hundred or less total crappie in the pond, you could stock more bluegill than if there are thousands of crappie present now. The big thing you want to avoid is exceeding the carrying capacity of the pond, which can cause a fish kill.
You could probably safely stock around 100 or so adult bluegill per acre. You don't want too many adult BG because they're going to be too big for many if not most of the bass, and young HSB, to eat and thus will be taking away food resources from the crappie; what you're after is the young bluegill that the adults will spawn and the crappie will eat.
Stocking BG will definitely provide a more reliable source of forage for the crappie, but that will make it even more important to have lots of predators in the pond. Otherwise the BG fry will take bass pressure off the crappie and make it very easy for the crappie to have a tonnage-caliber spawn.
