You probably could fill it, but not to pressures needed for paintball gun operation, CO2 tanks should not be filled with any gas or liquid besides CO2.
edit: you could use Argon in a HPA tank but not a CO2 tank
No, you must have it refilled at a paintball field or pro-shop. You cannot refill paintball tanks from low pressure Soda dispenser C02 tanks.
not argon itself, the cylinder may be because it's pressurised.
Yes, it can.
CGA580
A full cylinder is heavier than empty. The cylinder is filled by weight.
to increase splatter
Nitrogen, oxygen, argon, water vapor, CO2.
You must have a FM-200 cylinder refilled by a specialist company who hold the relevant certification to do so.
On steel a mixture of Argon and CO2 usually 75% to 25%
Mostly nitrogen and oxygen, with a bit of argon, CO2, water vapor and more.
For example argon or helium for TIG welding. Oxi-fuel welding: oxygen and a combustible gas: acetylene, propane, butane, propylene, gasoline, hydrogen, etc.
Whatever amount of pressure was used to compress the CO2 originally, can be reached (or nearly reached) when it is released into an air cylinder.