If it were the case of using an AT&T Galaxy SII Skyrocket, then yes if the device is unlocked as it can take advantage of T-Mobile's LTE networks due to both using the same frequency for their LTE network spectrum.
In this case however, yes you can if the device has been unlocked but I'm not really too sure if the SGH-T989 is compatible with AT&T's 3G/HSPA networks just yet.