Could we have the simple ability to create save animation poses to a pose library tab similar to Motionbuilder.
And have the ability to also mirror those poses and copy & paste them within the timeline.
This simple feature would go a long way to being able to create walk cycles and many animations Directly in UE4, without the need for external programs.
I m looking for the way to do this，but donot find any. Its any way have been find？ 
You can try the poses tool — it has a mirroring function.
But I’ve not been able to accurately mirror paste poses on any control rig that I built myself—if anyone knows how to make a control rig respond correctly to the mirror paste function in the pose tool I would be very grateful!