There is. First thing you do is choose "Symmetry" from the menu, and "Swap Left and Right" - once that's done, you'll have to manually change the Left and Right buttock bend, twist and side-side parameters (the symmetry button will not do this) and then change the side-side and twist parameters of Head, neck, chest and abdomen to the opposite of what they are (Negative if positive, Positive if negative). Finally, you may need to change the Zrot and Yrot parameters of the Hip or body to their opposites. Once that's all done, your pose will be 'flipped' and you may wish to save it to your pose library.