Anything can be done, the question is can it be done in a way that makes financial sense. If it uses a drum and niple, it is pretty simple to remove the drum and replace it with a vent. If the manufacturer has a flint model of the same rifle, and you can buy a flintlock to fit the lock inlet, you are home free. Then it can be done for the price of the lock and vent. If more work needs to be done, it will be cheaper to buy another rifle.