is it possible to take a 2nd ac compressor and make it into an air compressor to fill an air tank?

i don't know the insides of one...but it does move air/fluid around inside--right? is it possible.

except for the drag on the engine i guess...

Yes its possible. I have seen on other forums where alot of ppl do it. They usually use their regular a/c compressor though instead of adding a second.

The problem is that the bearings rely on the refrigerant vapor to carry oil with it. If you are just using it as an air compressor you will destroy the bearings before too long. There is a way around it but you have to add an oiler that will produce oil mist on the intake side, then add an oil seperator at the discharge side. Otherwise you will end up with alot of oil in whatever you use it on.