You could do it via soft-fork.  You just need to add a rule that the transactions for the first 50% of the block has a POS proof.

POS N:  validated by POS




POW N+1:




POS N+1:  validated by POS




POW N+1:




You would need to add a modification to the protocol to propagate the POS proofs, but that isn't a massive deal.

Non-upgraded clients would just track the POW blocks, but each POW block would need a POS block proof to be accepted by other miners.

One characteristic of this system is that hashing power is useless between the time a POW block is found and the next POS block is found.