Use IK to modify retarget animation result without custom rig?


After retarget animation, I can add an animation layer, and modify animation using IK in new animation layer

But the problem is that I have to setting up custom rig at the beginning.

If I don’t setting up custom rig, after I set Source to target character, I can only use FK to modify animation, which is inconvenience.
So I need a custom rig, but I don’t want to spend time to create my own rig, HumanIK built-in control rig is ok for me, so I try to use built-in control rig as custom rig.

But how can I do this?

After I click Create Custom Rig, Source is filled by Control Rig, If I select target character(B), then control rig become invalid, I can only still use FK to modify animation.

So how can I use IK to modify retargeted animation without creating my own rig?