Technically it can be done from what I understand (no explosions or corrosion or earthquakes flood or famine) but legally is another story. If you are in the US the legal answer is no. Any other country I have heard of the answer seems to be yes. The problem seems to come in when Joe HVAC tech decides to draw down your system and you have a mix in it. They now have all of their reclaimed r22 mixed with something the EPA does not regulate in R22a. One has to wonder if the EPA made money on R22a if they would say it was okay to mix in your system. R22 is so expensive now that many anecdotal stories are emerging of using R22a or even basic propane are out there if you look.