You are going to have to get a new hub/wheel to do that. I have replaced a coaster hub before. I suggest you find a old donor bike with a Shimano 3 speed internal rear hub and use that rear wheel. That hub is common and should be easy to find. The hub is quite reliable in my opinion and would give a few more gears to play with. Alternatively, you might find a single speed free wheeling hub with a wheel ..... or you could build your own wheel out of new components then get whatever you want.