My understanding is that  supercedure cell contains an egg that was laid to be a  worker and was  then repurposed to be a queen. So if the bees supercede  a bought queen they do not replace her with a swarm cell egg. The reason for the supercedure is then more likely the treatment of the bought queen (side effects of the grafting, being caged and starved during the transport?) 
Otherwise Mike S's comment can be improved: Instead of overcrowding you just take the queen and harvest the supercedure cells two weeks after the taking. 
-Jorg
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