I don't think you can.
Do Booted by releasing the hostages (will incur legion infamy, though, since I think you need to kill some legionaries), give Boxcars some Med-X, finish Why Can't We Be Friends in Vault 19 without destroying the vault (I ended it by blowing up the cave so Phillip Lem could convince the convicts to surrender and gained powder ganger fame). In all these cases the people you talk to aren't hostile. Note however that the best you can really do is a neutral rep if you're villified. Also, even if you raise yourself to neutral the powder gangers may still be hostile - as a group they're pretty bugged.
If all you want to do is gain access to their base put on a powder ganger outfit and take out the main gate guard from long range with a sniper (he'll sense you and everyone will start shooting) . Everyone inside should be friendly towards you.
- The named gangers in the admin building (and one in the cell block) can see through your disguise. Avoid the admin building and you'll be fine.