When you start to make a prototype of your game on UE4, you're trying to make the first iteration as fast and as cheap as possible.
Because of it developers usually use a lot of free unreal engine assets from the market place and also buy everything that fits the project.
At the beginning of the development, it helps to save a good amount of money and time but you still will face some problems.
One of these problems, that is pretty common and well known by me is that you have a big pack of animations for one character, let's say mannequin, but another character that you want to use has different proportions and different skeleton itself.
This problem might be solved by a runtime retargeting that UE4 supports, but in most cases, animations will look less artistic and can't be modified by animators
But in four easy steps, ue4 animations might be transferred to Motion Builder and baked to the character's rig, that will allow using all motion builder features for animations
01 Creating the MotionBuilder rig for Gadget
02 Creating the MotionBuilder rig for Dekker
03 Importing Dekker's skeletal animations and transferring (baking) them to Motion Builder rig
04 Merging Gadget's and Dekker's rigs, also retargeting animations
05 Dekker's animations on the Gadget character