What can you add to a voltaic cell to make it into an electrolytic cell?

a cell or battery

Explanation:

this is because a voltaic cell is such a cell where chemical energy gets converted to electrical energy. the energy conversion in the case of an electrolytic cell is from electric to chemical (battery supplies electrons to the anode respectively). So, I feel by adding a cell or battery, we can change the way energy conversion takes place, thereby changing the process.