It can be done, but is often more difficult depending on the gun you're starting with. If the gun has a drum and nipple arrangement you can remove the drum and replace it with a flash hole liner - that takes care of the barrel part. Then you need to fit a flint lock to the correct position to align with the flash hole. May or may not be easy to do - and may look funky depending on the lock used and stock configuration.

On some guns with a hooked breech it may bepossible to just get a flint barrel and lock to fit the stock.

All in all, it's a lot simpler (and probably cheaper) to buy the type of gun you want.