Why of course you can... you just have to know the rules, and play by them. Just like a 40mm grenade launcher, there are other options besides HE.
I had an RPG7 with a subcal unit that fired 7.62x39, but I traded that. I currently have an RPG2 waiting on the form 1/DD stamp. It's demilled in the traditional manner, but I will have that taken care of when the stamp comes back. I may build a subcal for it, just for fun, but I will build launchable projectiles (minus the boom).
FWIW, on the PG2, the 'rocket' isn't a rocket at all...it's a black powder launch charge. I should be able to get 'everything but the bang' as far as my rounds go.