You can connect external drivers, or motors with built-in drivers, to the pins on the expansion connector. It's simplest to connect the step+ and dir+ pins to one of the sets of step and dir pins on the expansion connector and connect step- and dir- to ground. Connect enable+ to +3.3V and enable- to the corresponding EN pin on the expansion connector.
Do not connect external drivers to the step/dir/en test points on the Duet.