It sort of depends on what you already have. If you have a schematic and PCB and want to add to it, that is easy - simply start a new page and add to it.
If you want to add to the schematic from another schematic, again quite easy - simply cut & paste. Same if you have two PCBs - you can simply cut & paste.
The tricky one is if you have two complete schematics & boards and want to "merge" them.
Maybe you could explain a bit more what you have (PCB/schematic) and what you want to add?
Keith.