Retargeting custom bones

I have a skeleton set up with some custom bones (a reference sync bone used to pair animations when grappling) and I want to retarget them to another skeleton using the Retargeting Tool. I am retargeting the main skeleton fine, but I don’t know how to add custom bones. There are advanced bones for additional bones, but nothing allowing me to specify my own ones.

Has anyone done this? Is this even possible? I’m so close to getting the results I need I really hope that I can get this final step because without it it’s a bit of a dealbreaker and I’ll have to figure out a different way to achieve what I need (ie without retargeting).

Thanks in advance.