you can do different dunks with players in practice mode according to their dunk package, so if you could control two different players in practice one at a time, you could have a dunk contest now though it wouldn't be spectacular.
for the contest use the database of dunk packages for the player to select, which means ea would have to specify which dunks are likely to be done in each package. and then either have the player perform a random dunk from that package after you make your move to the hoop, or ea could program it so that the player performs the specific dunk you choose from the package, making your control of the player more important in how well the dunk is done. all it really needs is a scoring system to be put in.