How to solve retargeting issues

I’ve been learning to rig and animate for a short time, and have lately stumped on a small problem: I’m trying to keep the animation-budget low by using one skeleton for several models with different proportions. The animations of this one skeleton are restored by using retargeting, but the hands and feet of the characters never end up in their supposed places, like on the handles of a bike.
The documentation from unreal states this can be solved by rigging extra IK handles for the feet and hands, and using those to have the hands and feet remain in place.

I’ve used the ART-tool for Autodesk Maya to rig some of the models, but I’m lost at the point of adding these extra IK handles. And after that, how do you switch between the IK and the FK like mentioned in the documentation?

I’ve tried to look up as much as I could, but either this is a really simple matter or it simply hasn’t been discussed yet. I hope someone can take some time to help me with this.