A responsible gas supplier will not fill that with CO2 for you. For one thing, none of the threads on his equipment will match up. Also, the CO2 is under many times more pressure than liquid propane. The cylinder walls on a CO2 canister are much thicker than on a propane tank.
Just don't try it. It isn't safe. It's not worth getting hurt/killed to save a few bucks.
Propane in a tank with liquid propane in it is at about 130 psi pressure at normal room temperature. CO2 in similar conditions is at about 800 psi. That's why a propane tank is not the same design tank as a CO2 tank.
bottom line, propane tank is for propane, co2 tank is for co2.
Go to your local AutoParts place, they normally have an exchange program for gases. Locally here I can get a 20lb bottle for the first time for 160 full of gas, that's a 130 core, when i am done with the bottle I get my 130 back. So its only 30 bucks for a 30lb bottle basically.