I believe it’s possible to create a skeleton from scratch using the Apex Auto Rig Builder:
(https://www.sidefx.com/docs/houdini/character/kinefx/autorigbuilder.html#buildrigwithoutskel)

How should I assign weights to a skeleton created with this Autorig Builder?
The skeleton and rig are generated successfully, but I can’t figure out how to bind this newly created skeleton to geometry and apply skin weights.

Folder unpack the Base.shp and the Base.skel, do the weighting and folder pack the geometry back into the character stream