Answer:
Step 1: duplicate a human skeleton in openBRF, modify it and and rename it.
Step 2: open skeleton_bodies.xml (inside data folder) in a text editor, copy-paste human skeleton there (it's some easily identified 20 lines), change its name to match the name used in step 1. XML is easy to edit (it's like HTML).
Step 3: in you module_skins.py, duplicate a race into a new race, make it use the new skeleton.
Step 4 (optional): right click on the new skeleton, pick "add it to reference skeletons"; that way you will be able to use the new skeleton in the future to preview your animations and your rigged objects.
These steps are needed even now to add a skeleton.
(If you omit Step 2, it has the weirdest effect: your new race will have no hitbox so it cannot be hit at all--this might infact be an usueful trick for modders, to grant invulnerability or the like).
Nothing of that has anything to do with hitbox customization. These steps were needed before, and will be needed even after (and if) I'll make openBRF customize hitboxes and save them in XML. That file includes also other information (for ragdolling, like the fact that knees bend in only one direction etc), so it is needed anyway.
Now the customization part: open brf (next version), see the new skeleton (or an animation and set it to use the new skeleton), check "view hitboxes", edit manually xml file, hit F5 (refresh). OR, you might want to wait for the version that lets you edit them more intuitively in 3D, as described 2 posts above.