Assuming that you do not have access to Copy Geom functionality I would suggest the following.
Create a copy of the surfaces you want to reference from the imported STEP solid geometry.
Determine a Csys that you can align to your derivative model you may want to create a new one rather than using the default Csys.
Export a STEP file and select only the newly copied surfaces for export.
Import the STEP export of your surfaces into a new part and use it to design your new part.
Guide to export a quilt using STEP interface, it is not intuitive in the UI.
Involute Development, LLC
Specialists in Creo Parametric