What you describe sounds like a superelement, which is referred to as a Condensed Part in Mechanical. In recent releases this is a fully supported capability in Mechanical. You can right click on Model at the top of the tree and insert Condensed Geometry. Then right click on Condensed Geometry and insert Condensed Part. After scoping the Condensed Part to some geometry you can right click on it again and Detect Condensed Part Interface and Generate Condensed Parts. All the places where the box connects to the support frame will become master degrees of freedom of the superelement.
You are navigating away from the AIS Discovery experience