I'm a Boardstation/RE user and have never used PADS and don't know the official answer,
but since no one else has taken a crack at this question here are some considerations:
- Since the layout is driven by the schematic, it seems like you would have to combine the schematics first
- I'm not sure if PADS supports blocks or re-useable circuits, but maybe you can plop the individual pieces into a design using that kind of functionality
- If you just need them combined for manufacturing, you may want to find out if you can combine layouts on a fabrication PANEL or assembly PALETTE design, if your software supports that
Again, I'm not familiar with PADS, just trying to help,
Hope you figured it out by now but here goes.
Two methods I have used in past. Both need layer counts to match. electrical and documentation.
Asci out board to copy without board outline then import into receiving board. refdes name and positions will remain same as copied board. Net names will be preserved.
Copy and paste
Open board to copy. Select parts, traces, vias. Move curser to 0,0 then copy. Close board Open board to paste into. Activate ECO mode. Paste will occur at cursor location. All pasted refdes will increment up from existing. Be located relative to curser position. All pasted nets will be added as $$$# generics.
Could also be done with reuse, but I have not used that enough to know what you will run into or how to set up.