Oh, I know -- split your party. Engage one of them in conversation with Roah, for trading or whatever, and have another one climb through the roof to get into the room while the conversation is going on. She won't be able to retreat.
The retreat happens off-screen, and once she is inside the treasure room, she stays there.
Waiting for patch 4 to drop before I reinstall the game, so I can't check this method for you.
Since you think you should be able to talk to her, I'm assuming Roah isn't directly aligned with the goblins. If this is the case, sending Halsin in the room won't trigger hostility on her part.
Rescue Halsin. Leave at least one Goblin boss alive. Get him to the treasure room. Either clear out the front area, or if you're sneaky, and the goblin kids didn't escape the worg pens, turn him invisible.
Shove on level ground doesn't do damage, so Halsin should be able to shove Roah without triggering hostility.
There is also a possibility that Halsin can throw her as well. Some NPCs(like the Gith in the mountain pass, and Kagha) can be thrown by Halsin without triggering hostility, but you'll only be able to do it a limited number of times before killing her.
If her AI is passive, and she doesn't try to walk back to her original location after shoving, then just shove her until she's out of the room.
If she tries to walk back you'll need to do some prep, and partially block the door by shoving barrels. So that when you give her the final shove, you can walk forward and Halsin's body will block her path back into the room. Entering turn based mode will make it easier to make progress shoving her out the door.