nitrogen filled tires.....how do you add?? is air OK?

I'm continually alternating the rear tires on my truck between 50 and 80 psi depending on whether I am towing or not. Not about to go looking for nitrogen. Depends on the humidity where you live I guess but I get little or no moisture from my compressor's tank when I check the drain. I painted the inside of my trailer rims with calliper paint so I am not concerned about the wheels corroding internally.