Yep, as long as the new SPC is added to a higher-numbered slot than an existing one. The lowest-numbered slot will have one SPU acting as the CP (central point) processor, which handles session setup, internal load balancing, and stuff like that; if you add a new SPC into a lower-numbered slot of the backup, then CP on node 0 won't align with CP on node 1 and sync will fail. If the SPC is added to a higher-numbered slot, then sessions from the exisitng SPCs on node 0 will sync to their matching SPC on node 1 (with no sessions sync'ing to the new SPC), and you should be able to fail over to node 1 without any excitement at all while you add the new SPC to node 0.