Buy phone at retailer and have ATT Next go on existing plan?

I'd like to take advantage of the Black Friday deals to upgrade my phone. We currently have two fully paid off phones on AT&T mobile. I'd like to buy a new iPhone XS at Walmart to get their gift card. The ad says it must be a ATT Next contract. The question is, does that have to be a new ATT account or can they just put the phone on my current account as if I bought it through ATT? We don't want new accounts, we just want the ATT Next payments to be on our existing account.

Thanks