Currently we do not offer our packs pre-mirrored but several have been created by using relation constraints inside of Motion Builder. You can follow the following tutorial and pictures.
1. Add the "root" as a Character Extension to the Character, so it will plot with the Character. (I am also using it as the Character Reference)
Bake to the Control Rig with the Character property "Mirror Animation" turned on. If you want to batch many Takes, you can plot all at once.
2. Turn on the "Root Mirror CtrlRig - Relation". It will mirror the root backwards just like the Control Rig.
3. Turn off "Mirror Animation". Plot back to the Skeleton. As before plot all takes if you like. Turn off "Root Mirror CtrlRig - Relation".
4. Takes will be mirrored but facing backwards. Turn on "Root Mirror Skel - Relation", Character will face forward correctly.
5. Select the "root", then select translation and rotation in the properties window. Under "Animation" select "Plot Selected (Selected Properties)...".
6. Turn off "Root Mirror Skel - Relation".
While this constraint is setup you can use it across multiple takes by plotting each one before finally turning off the constraint. Also euler can get bug out sometimes so check your result. The basic principle of how this constraint works can be used to achieve the same results in other software.