My in laws are going to be flying PEK-CLE sometime in April. I'd like to buy them an economy fare and then upgrade them using the co-pay and miles method. The problem is, they don't currently have onepass accounts and of course they have no miles. I'd like to use miles from my account to "pay" for their upgrade. Is this possible, or is there some restriction on that?

I'm sorry if this has been asked before, my searches didn't yield the correct results.