Ok, after reading your cringe-worthy horror description of reproduction for children I decided to take a look at the OP again.
I think I know what your problem is. BUTT! That is only a problem if your extra robe and armor joints are actually animated in your custom source skeleton.
If not, you dont have to care, just retarget the base skeleton and all those extra joints will just follow their parent bone (which hopefully is part of the base skeleton).
But I think thats not what you are after, let me recap so I can make sure this IS your problem:
You have a hunter character rigged with your own custom skeleton. That however needs to be ue4 skeleton so it can go on the marketplace.
You will have to use the epic skeleton for that character wherever you can. Generally that is arms, legs spine, head, fingers, and the extra leg and hand Ik bones.
For your additional armour and costume bones, you will need to manually add those to the new epic base skeleton and retarget those animations in some other software. (motionbuilder comes to mind).
And yes, you are right, in this special case epic retargeting system is lacking more extra bone slots since you are retargeting from your own old skeleton to the epic skeleton that you made WITH extra bones from the old character skeleton.