Creating Rigs for Free assets as Marketplace products?

Is it acceptable to create a collection of animation and/or IK retargeting rigs for existing “products” that are in any of these categories?..

  • Epic Games Content?
  • Permanently Free Collection?
  • Marketplace Collection?

There are quite a few Skeletal Mesh assets in these collections that have no rigs included.
Theoretically, rigs could be created for them and submitted as a marketplace item with rigs and stand-in skeletons (no meshes).

Likewise, animations could be authored for these assets.

Would submissions of such control rig and/or animation assets be acceptable?