There is no reason you can't use 120 in a 220 back. The thing wiht the preasure plates is a load of nonsence. The plate is floating on springs and so are the rollers. The film plane is in the exact same place as it is for 220.
You'll notice you will be using twice as much film now with 120 as it is half of a 220 roll.
Oh don't forget you have only 15 shots now and can stop there.