There are a few possibilities;
Depending on what speed and torque you need, you may be able to connect two stepper motors in series (which is how the two Z motor connectors are wired) or in parallel, if you don't need to control them independently. Using 24V power instead of 12V helps. Use the motor calculator at reprapfirmware.org to see if this will work well with your chosen motors.
If you are using any external stepper drivers, you can connect motors in parallel to the external driver outputs if they have high enough output current capacity.
Also if you are using external stepper drivers, you can connect stepper driver inputs in parallel.
The above methods don't allow independent control of the motors that are driven from a single driver channel. To get independent control of more than 4 motors, you could modify the firmware to reallocate some of the pins on the expansion bus to drive additional stepper drivers. Somebody already did this to drive 14 motors independently.
The advantage of allowing independent control of each Z motor is that you can use them to to bed levelling. See https://duet3d.dozuki.com/Wiki/Bed_levelling_using_multiple_independent_Z_motors.
For the future, the Duet 3 as currently prototyped provides 6 stepper drivers on the main board, then you can daisy chain expansion boards that carry 3 drivers each. So the main board plus 4 expansion boards would provide 18 stepper drivers. Or the main board plus 3 expansion boards would provide 15, and you could use the dual outputs on the main board to connect the extra motor.
