I have 2x AP11 locked to Asia Pac region.
Can I add on more AP11s that are sourced from US region? Will they be able to join to the existing network?
Would there be an incompatibitily issue due to the slightly different wifi bands allowed?
AP's with country code US are locked and can't operate in other regulatory domains. Same goes for the other regulatory domains; Japan (JP), Egypt (EG) and Israel (IL). All other countries can use the Rest of World (RW) SKU's and you'll be able to select the corresponding country in which the AP will operate.
My suggestion; buy/use AP's in their respective region. Every region has multiple partners that sell AIO https://www.arubainstanton.com/buy-now/