Lets say I have a model with a ready humanoid rig and a humanoid model with no rig. Can I somehow copy the bones from the rig to the model with no rig in unity?
This is possible, but only if the model has it's vertex groups defined, with the same bones as the original. you could do this pretty fast in blender by just parenting the model to the rig. select the original model, and shift select the new model, go into weight paint mode, and click transfer weights. But i don't think unity itself can automatically assign weights to the mesh.