You have your widest aperture and your highest ISO in play already. To brighten up these subjects the only tool left at your disposal is to add light unless someone has some better ideas. The problem with a flash is that you run the risk of blinding the dancers. It would be better, if you have any influence with the dancers, to have them add a few extra lights to their show.

The only other thing I could think of would be to catch them at points where they are more at rest and use a longer shutter speed.