You could try this. Work on the base geometry obj of the hair. If the hair uses internal geometry you will have to export it as an obj.

View with the front Camera. With the hair obj in Poser, change to Outline Display Style, and Paste into Background, change back to normal display style. xScale the hair to 0.1 (Poser won't go any lower). Export the scaled obj. Load it as a morph target into the original hair obj. Set the morph to a value of 2.0 (it will look strange at this stage). Open the Joint Editor and use "Reverse Group Normals". The hair should now look normal but mirrored. At this stage you may, or may not, have to xTranslate the hair slightly to be centered on the Outline Display image.

Save the hair obj with _Mirror appended to the original name, eg "KyokoHairMK3_Jessi_Mirror.obj"

Make a copy of the original hair hr2, (or cr2 whichever), and change the object reference to refer to the new "*_Mirror.obj" (figure hair will have two obj references that need changing). With a bit of luck it should work.

For all obj export, use the export settings as in the above image. For obj imports, make sure all import options are un-ticked.

P.S. Which hair are you trying to mirror?