You brought that STEP file into either DesignModeler or SpaceClaim.

In either case, just bring the new STEP file into the same project into DM or SC.
In DM it is Import External Geometry File, in SC, on the Assemblies tab, click the File button.

Now you have two bodies instead of one.  Refresh the Model and open Mechanical.

In Mechanical, you can select each load and support and change the scope from one body to the other, then suppress the original body.