Every CO2 bottle is filled by weight.

Therefor it's easy. With a cheap filling station, you're decanting liquid CO2 from one bottle to another.

This requires that the recieving bottle is a lot colder than the source bottle. Not difficult, after a fight a CO2 bottle from a robot can easely be - XX°C

You know the weight of the empty bottle. It should be stamped or printed on the certified bottle. If not, be sure you mark it clearly.
Paintball bottles come in oz , meaning 30 grams per oz. A 20oz bottlme therefor can contain 600 gram of CO2.

You connect the 2 bottles with the filling station, and you open the valves in the right progression. You'll hear the transfer of liquid CO2 happening.

If the recieving bottle was cold enough, the audible part will tell you when the bottle is overfilled.

Messure, and dump the excess. If you don't, the burstdisk will let go when the bottle heats up.

If too light, dump the bottles content, that will freeze the bottle, and repeat the procedure.