It's locked to Boost/Sprint.
I suppose that if you were knowledgeable enough to hack it (SIM card?), you wouldn't be asking. I know nothing about that sort of thing, myself.
It is sold without a prepaid card. You have to sign up for some sort of Boost service to activate it. (One plan from Boost: $50 for 10GB or 1 month, whichever expires first. No rollover of any unused data left over at the end of the month.)
If you're an AT&T customer, do you have a 4g smart phone? Is it enabled to function as a WiFi hotspot? Data through a regular cell subscription ought to be less expensive than a prepaid service.