You can fill those only if the 20# has a dip tube in it which has 0 chance. When we fill the our CO2 it is filled from a huge tank (obviously) that feeds our lines from the bottom. Though, you could turn the 20# upside down and feed it to the 5# similar to siphoning. But after all is said and done, just bring it to your local guy, or Airgas
(who do I work for?!?!) and get them filled, I don't pay for gas but 5# tanks are cheap to fill, should be $15 or less.