Imaginary friends are occult sims, even when they've been made real (and they can still transform into a large doll when made real). The only ways to remove, change or combine their life state involve using mods : MasterController with the Cheats module to remove and replace their life state, and Hybrid to combine two life states. Be careful with some combinations with Hybrid though, as some hybrid life states can be bugged even with the mod (which isn't the mod's fault, as there is some hard-coded conflict between specific life states in the game itself), so keep in mind that it's experimental. Changing life states with MasterController alone is safe as long as only one occult state is active at a time.
