Yes you can buy them while in the states. Either logging in to the account or if you connect to T-Mobile (which you need to anyway to get anything to work) then you can also dial 611. Or the toll free number 1-855-4pu-blic.
Data should work fine as long as you're in a coverage area. You'll get texts. You can't send texts or call either direction without those other roaming add-ons.
Be sure to turn on the roaming data option on the phone (and of course data).
There are a few USA add on that you can purchase. Read the warning first
A bit of pre planning for you.
1. If your account renews while on vacation, prepay your next payment before you leave.
2. Find a voip app to make calls on over wifi.
3. Find an alternative USA prepaid carrier. Make sure you have an unlocked phone.
Why did I say that.
1. The payment set up on a credit or debit......if that card goes awry when away then your PM account may get suspended if card does not work. And every time “if” mine does...it’s in the USA.
2. Connections are getting unreliable in USA....so have an alternative. Texting inbound is still “supposed” to work while you are in USA though.
3. I have used Tracfone and bought SIM at Walmart. And others report that Mint mobile is good too. Not sure where to buy a Mint SIM.
I will be in states for 18 days but only interested in add ons for the last 10 ( where I won’t have wifi ) so I was wondering about adding the add on at that time , from within the states. It also sounds like what I need is data more than talk text - so getting the data add on may be way to go! Thanks !