I don't know about the preorder, as I have never ordered through Best Buy. But you can just add it later through Apple up to sixty days after purchase. It shouldn't matter that you purchased it from Best Buy, as they check your serial number and inspect the phone.
AppleCare+ is only available for use with Apple (in-store or mail-in). No need or requirement to take your iPhone to BestBuy for Warranty or AppleCare+ repair service.