Yeah I want to do this too. Unfortunately I didn't have enough stardust nor rare candy yesterday.Lucked into spotting the news of the cheap secondary moves on legendaries. I managed to add Overheat to my best, powered-up Sky Attack Moltres. That was the one that always tempted me, but 100 candy was waaay too steep.
It's a 98% ine on lvl 40. I'd really like to have a fire charge move on it