Can a person become a buddha after reaching nirvana?

There are answers here that suggest that being a buddha is not the same with entering nirvana. Many people enter nirvana. Very few become buddhas. So I wonder, say you enter nirvana. But then you want to "complete the mission", like replaying the whole game. Can you do that? Let's say there is some bodhisatva that doesn't enter nirvana until he accomplish something. So after he enters nirvana he can't accomplish what he wants to accomplish? Are there buddhas that do not enter nirvana?