yes. you can do it.

the only prep you probably have to do is to make sure surface is free of oils, etc. test some alcohol on an area (bottom bracket) to see if there are any adverse effects. acetone would do it too, but that may be too strong for powdercoat.

you can clearcoat over pretty much anything, so you should be good to go when you're done.

edit: on second thought, just clean the frame up with soap and water. disregard my alcohol/acetone suggestion. make sure you rinse it clean and use gloves so you don't get any oils on your frame before you paint.