you can copy-paste the values of your front bone to your back bone as long as your axes are set to world or local. just select the first bone, hit ctrl+c, then select the second one and press ctrl+v.

If a walk is frontal and you want to literally mirror the position of the bones, you could:

• duplicate the bones that you want to be mirrored with their keyframes,
• create a new bone with rotation set to 0 and parent the bones to it,
• set the scale of this bone to -1, 0,
• then go into your flipped bones and copy the position into your to-be-mirrored bones.
• you can delete the new bones you created afterwards.