You need to use a spare pair in the existing detector to carry the circuit back to the panel, panel has 8 zones so if full keypad has a home for two more which can be carried back via spare cores if available. If not your need an expander to bump the system up by 8 more zones. System can take up to 24 in total via expanders and keypads. You can share the power by piggy backing off the existing detector, it’s the two circuit wires that need carrying back to a Zone somewhere.
