Group Selected option doesn't work in Isaac Sim for mesh components

When I import complex 3d object that is composed from multiple sub objects into Isaac Sim the object hierarchy gets flattened. I cannot fix this afterwards also - group Selected option do not work and I cannot drag sub objects under XForm component created specifically for this purpose. How can I deal with this when my end goal is to have the same object hierarchy that I had in the original model or close to it.