You can connect 2nd gen Iris keypads to the system (or any other zigbee/zwave keypads for that matter). To get these to interact with the ADT hub for the purposes of arming and disarming the system, I believe you’ll need to use something like the ADT Tools 2 smartapp, found here: https://community.smartthings.com/t/released-adt-tools-2-for-smartthings-adt-alarm-sytsems/124951
You’ll also need a DTH for the keypad you want to use.
Please note, when set up this way the keypads will be cloud dependent, which means they won’t be able to arm/disarm the system if you’ve lost internet or if the ST cloud is down.
Another popular option is a spare tablet or smartphone running something like ActionTiles or another dashboard mounted to the wall or in a stand in a convenient location. This can be used like a keypad, and integrated with the ADT hub the same way. The benefit is that it allows for viewing and control of much more than just arming/disarming the system.