In this video i show how to mirror an animation for UE4 using Motionbuilder and Maya. The motioncapture animation needs to have a solid t-pose in order for this to work. So if your animation does not have that, i don't know how to handle it. I will probably see if i can find a solution for this, since this could prove usefull later on.
The character i use in the example here are from Mixamo and i don't show the specifics in Motionbuilder how to match up the joints but will maybe make a video about this later.
Let me know if you have questions.