It depends of the bones hierarchy.
Even if the bones doesn’t have the same names in both skeleton, you can use the humanoid rig to match your skeleton with the SK_mannequin… But you need to know if the bones are arranged between them in the same way the SK_mannequin is.
Did you tried to select the SK_mannequin skeleton while an import ? Maybe it will work.
If not, you’re not screwed… You’ll need to retarget your animation for the skeleton you want to use with them.
I think that the MoCap software is meant to do this but I don’t know it for sure.
You should open your FBX file with any software to see how the bones are attached to themself.
Eventually, add a simple mesh and skin it to some bones. Then, import the new FBX (with a mesh) in UE4. Make the retargeting from your skeleton to the UE one. You will be able to import the other animations on the skeleton you retarget to.