Given 2 nodes, 2 existing shelves, R5 set size of 6... assumption is that your cabling follows the best practice for 2 nodes 2 enclosures in:
http://h20628.www2.hp.com/km-ext/kmcsdi ... 7590-4.pdf(Meaning that each enclosure is on a separate SAS loop)
I don't believe adding disks equal to the set size will matter. What currently is impacting us most is the the number of drives that can be bought at once. I am currently being told that I can only buy drives for my 7200 four at a time. I don't believe this is a technical requirement, perhaps a licensing or bad marketing. Ideally, you will want your new spindles split equally between the shelves, assuming they are cabled per that doc I linked and split between the 2 SAS ports.
Adding just 1 disk shelf is supported, but I would prefer to add 2 at a time to keep both SAS ports/loops "balanced".
Probably most important is to rebalance the system after adding more capacity, I believe you should have access to systune by default on the 7000s. This will redistribute your existing data across all the spindles new/old.