@Bill_Becher, can you please clarify your question? Is it how to add music to a Nucleus or is it a question of why Roon Remote and Roon ARC have different capabilities?
Does your Nucleus have a internal storage drive installed?
If this so this guide should help …
If not are you considering adding one?
If not are you considering adding an external drive USB drive?
If you go this route you can copy over files from the PC to this storage, see the help page for details.
If you do not wish to have music storage on the Nucleus, then the only alternative to the about would be to setup a watch folder that points to a SMB folder.
This shared folder in your can would be on PC, but it could be on another network device such as NAS.
Personally, for simplicity and ease of operation I’d opt to music storage on Nucleus (and have the PC storage for initial CD ripping / downloading and backup).