Hi
First, sorry to my English
I will use motion capture in unreal engine. Motion capture works well with biped in 3dsmax because i import .bip but not with the CAT’s biped.
The root motion in Cat is nice to follow the Bonne Bip01, but with the basic biped, there is not this option.
Someone find au solution to create a root motion with the basic biped in 3dsmax who follow the Bone bip01 ?
I am not sure if i am clear 
Thanks
My goal, make a “motion extraction node” like CAT.
You can, It is the same way you do it for CAT. What I mean by that is that:
1 - In CAT Never use the default method and root bone. It will bug out on you at some point and can be unpredictable with less control.
2 - Build your own rig setup that follows the CAT or Bipeds bones, Build each bone linked as a hierarchy, then link or rotation/position constraint them to the CAT or biped bones. then export the custom rig not the biped or CAT bones.
3 - You need to use a dummy object as a root bone and create a root motion setup that will allow the dummy to move in X and Y axis following BIP bone and not Z axis. As well as Rotate in those axis.
To build this you need to make use of the Motion extractor helper node in Max and wire parameters or Script controller (preferably the latter) to connect position movements because Biped and CAT work in quatornian space and Cats controllers are extra specific to it.
The setup could take a while at first but once done right its there to stay, DON’T take shortcuts when rigging it will bite you later.
here’s a link to help out.