NA6005 also has a dac
And 3 digital inputs
It's a network streamer cum dac but by using volumio you would take it's network streaming out of the picture which should be OK depending on what hardware you use to feed digital data to it externally
I don't think you can directly. It is not an endpoint. You need to add a device such as a Raspberry Pi with Volumio installed on it. Add a board which outputs digital signal like Optical or Coaxial. Feed that to your Marantz digital inputs if it has. If not, then you would need an external DAC which then needs to be connected to the Marantz.