I know how to do it, but havnt done it as yet. Changed oil on dhx's and the basic principle isnt any difference. Big difference is that dhx's have a schrader valve to pump air into, fox air shocks dont. They use a rubber pellet compressed with a grub screw..the russian guy managed to make some needle device fitted to a shock pump to mimic the fox nitro charging tool.
Air would be fine instead of nitro…the water content in air is the reason they use nitro, so a nice dry day and you'll be fine. Pressue wise fox upped the pressure from 300psi to 400-500psi a few years ago as they had problems with larger riders bottoming too easily…300psi (normal shock pump pressue) wouldnt be a problem unless you're heavier than ~14st and ride hard.
Only reason i havnt tried it yet is my shock isnt in need of a service…if it was i would have a bash and if it failed/poor performance then i would send it off…would be no more expensive for the service.