The Duet 0.6 supports up to 4 external drivers, so in theory you could buy a bare RAMPS PCB and add the sockets for driver modules, the electrolytic capacitors and the microstepping select jumpers (or hard wire them). However, the firmware for older Duets doesn't support configuring drivers via SPI or UART. So unless you want to make extensive changes to the firmware yourself, you would be limited to using a fixed driver configuration and you would have to set the motor currents using the potentiometers on the driver modules.
