thank you very much for your question.
Just enable the "Mirror" flag in the Inspector of the exported *.anim clip. In UMotion, you can only mirror the animation of bones that have a sibling on the other side. E.g. you can copy the pose of the left arm to the right arm, but you can't mirror the spine's pose (as it has no bone on the other side).
Further information: UMotion Tutorial - Pose Mirroring