Yes, any nitrogen tank is refillable.
Most paintball markers can hold a N2/Nitrogen/HPA/Air tank so I couldn't possibly list them all, if you submit specific markers I could tell you if they fired using Nitrogen.
if you have your own gun, probably cheap
you cant use it or refill it
A compressed C02 tank or High Pressure Air tank propels the paintball.
There is not 100% safe air unless it is 100% empty. Nitrogen i would think would be more safer due to the gauges, and the carbon fibers spun into most (depending on the tank). Though, keep in mind, nitrogen is $100+ more expensive then CO2
Where the air tank or co2 tank normally attaches
your local paintball field. Or you can purchase a scuba tank and adapter to fill the paintball tank with and every once in a while fill the scuba tank up if you live like out in the middle of nowhere
Take it to an airsmith or paintball tech.
No.
High pressure air, and all air for that matter, is about 78 percent nitrogen, so it is nicknamed a "nitrogen tank" . So a HPA tank is supposed to be filled with "nitrogen" which is really just air.
No, paintball co2 is much higher pressure.