I'm sure I've mirrored props with a magnet. Make the MagZone huge eg 1000% to avoid distortions. Set X-axis scale to -100% (or was it -200% ?) Will need to export/import object and save as a prop.

Just tried it in Poser 8 but it won't scale below zero despite altering the Min Level settings :(

Poser 11 , 180Gb in 8 Runtimes, PaintShop Pro 9
Windows 7 64 bit, Avast AV, Comodo Firewall
Intel Q9550 Quad Core cpu,  16Gb RAM, 250Gb + 250Gb +160Gb HD, GeForce GTX 1060