Most people go the other way around, i.e. add to internal drive and backup to NAS.
If you don’t want Roon to access files on your NAS, then you don’t need that in Settings==>Storage.
The music on your internal drive is accessed by Nucleus Internal Storage (something like that. I use ROCK, so I don’t know the exact name for Nucleus) entry in Settings==>Storage. That is given to you when you have an internal drive connected. You don’t have to add it.
Nucleus exposes a network Share. Use another device to copy your music there. Here’s mine on WIN11. You want to copy to the ‘InternalStorage’ folder. -
