Many have responded that you wouldn't be able to bank and added comments about being within 30 days. I was under the impression that under a purchase/add-on scenario the banking deadlines were waived for the purchase/add-on if you were already within your banking window. It doesn't make any sense that you would not be allowed to bank the full amount of your points extremem example but if you had 150 (original and banked) and added 300 and you were within the 50% bankinig window...you are all saying that you would potentially lose 75 points (50% of 450 total =225, but you have 300 to bank and only allowed 225 at this window)
Hope that's not too confusing, but at the end of the day it makes no sense because you'd potentially be at a point where you wouldn't be allowed to bank.
The obvious way around it is to do 2 add-ons, to avoid the banking window issue, but like I said before, I think the banking windows are "temporarily" waived when you do an add-on/purchase, so that once your contract closes you'd be allowed to still bank those points.