If you want to give it a try, this is what you will need to do:
Get the latest RADDS version building, using the dev branches of both RepRapFirmware and CoreNG. There are build instructions for the Mac in file buildinstructions.md. I didn't write it, so it may be a little out of date now.
When that's working, switch to the v2-dev branch of RepRapFirmware and the main branch of CoreNG. That's where development is taking place at present. You will need to add the FreeRTOS and RRFLibraries projects, and port some changes into the RADDS-specific files in the CoreNG and RepRapFirmware projects. I can give you advice on that. You will also need to add a SAM3X build configuration to project RRFLibraries.
We haven't supported RTOS on the SAM3X-based Duets because of shortage of RAM. However, as Due/RADDS does not have a network interface, there should be sufficient RAM saved by not having the network stack.
