You can receive texts in the US but to send them then you'll need roaming add-on. To make calls and use data will also require a roaming add-on.
and Purchase when you visit to USA not now,
sign in to Self-Serve, to review your account,
How To Purchase Add-Ons Using Your Self-Serve Account
To purchase Add-Ons using your Self-Serve account, follow these steps:
after you Add-on you have to do Rebooting your phone
what is mean for Rebooting the phone means to turn off your phone and turn it back on again.
Overview Of Add-Ons visit Here link.