drywall over cedar shakes? Can I do it?

I have a 3 season porch that I have recently sealed up with new windows and a new solid wood door. Whoever built it put the same cedar shakes that were on the outside of the house in this room. As I am trying to make it into a 4 season porch I want to put drywall up. Can I just drywall over the shakes instead of removing them? Any help is much appreciated.

thanks,

PC