DAZ Studio 4 Pro can natively mirror poses! I think other versions of 4 (Advanced, Free) probably can as well.
Load up Genesis and apply a pose. Make sure you have Genesis selected.
The dialog in the above picture will pop up. Choose the selected options.
If you click on the Preferences tab, which you can't see until you hover over it (name right next to the main tab), you can set these as default options and not have to change them every time you mirror a pose.