If you can come by a pump of some sort, remove what's in the resivoir then fill it up with fresh fluid. You'll probably be fine to drive it a couple days until you can flush the system. The way I always do it is just bleed the brakes until you go through one of those big bottles of fluid and all the stuff coming out is clean. I always use Valvoline synthetic these days. You can buy it at Autozone and probably Wal-Mart.

The only big problem I can think of with PS fluid is it won't handle the heat that brake fluid does. Somebody else might know if there is a chemical reaction to worry about. I've never heard of any such thing.