There are two ways that I would consider doing this.
1 load the rig with all the FX than you want to use. Then Lock Everything except the Amp and Cabinet sections. Next load the rig with the amp you want. The amp and cabinet will be loaded but all the locked FX will remain from the first rig. Finally save the new rig.
2 drag the amp you want to insert into the Preset section of RM. Now load the rig with all the FX. Drag and drop the amp from presets into the amp block.