I flew through a young cumulus stage cloud one time that looked similar to that in a Turbo Arrow. No rain, but the turbulence was so bad that full control deflection wasn't keeping us straight and level. There was a DC-10 near us on the frequency, he had enough of the turbulence and diverted to Detroit, which didn't have any thunderstorms. When we were in the updraft, we were going up at about 1,500' minute with no power and pitch down about 5 degrees and we started picking up ice, quickly. I let ATC know we were deviating from our assigned altitude and I will never forget what he said, "You are the only aircraft out there, so you can do whatever you want." Talk about a lonely feeling. We were out of it about five minutes later. You only need to experience a thunderstorm once and you will always remember that you never want to do it again, I did not make that mistake twice.