Altium: how to explode an imported 3D model
You would be much better off using a tool like SolidWorks or SolidEdge to do your mechanical work. Altium is not designed to handle enclosures. You would export your board from Altium as a STEP model and open it in the mechanical CAD software. If you use SolidWorks, you would use the built-in tools to separate the pieces of the step model, and you would put your board STEP model into it. You should not be doing this in Altium.