Can an existing painted metal roof be stone coated?

I do not believe there is a way to field apply stone coating on an entire roof.

There are companies around the world that can field apply stone coating to existing stone coated roofing with limited success (carry a 10 year warranty), but that is not a practice that has been really used in North America. There are paint systems that also have limited success (also about ten years of warranty) such as NuTech that have been used to change a color for old stone coated roof systems.