You’d need something to convert the output of the Nucleus to the digital inputs that the DAC accepts.
There are USB-to-S/PDIF converters in the market. Alternatively, you could use a Roon Ready network streamer that has an S/PDIF output; it would read the Nucleus output via Ethernet.
You might want to take a look at Matrix Audio X-SPDIF 2 or 3 which can be connected to the Roon Core via USB and offers all possible kinds of digital outputs incl. AES/EBU as well as very good internal clocks.