Airbrush compressors are only rated up to a point where most airbrushes require. I don't think a d500 will push more than 30psi. 30 psi in a tank won't last you very long, whether it's 7 or 70 gallons. You could keep the pump running to keep the tank full but that defeats the purpose.
You're better off buying the right fittings and filling the tank at a gas station. You could also just use the D500 the way it was meant to be; nothing wrong with that.