WARNING placing the SSD into the Nucleus it will need to be formatted for internal use and anything on it already will be lost, so you need to be sure if you use the same SSD you have on the external USB that your music is also backed up elsewhere.
The Nucleus supports both Internal Storage and External Storage. If you have installed an internal SSD (in addition to the one that came with the Nucleus), then you can add content to the internal drive via SMB, more information here:
If you want, you can also leave the attached USB drive as a storage location, you’ll just want to make sure you add it to Roon Settings -> Storage as a watched directory, more info here:
Did you install an SSD in your Nucleus for the purpose of storing your music files? If so, you need to follow the instructions to copy your music files to that internal SSD. Then, you need to make it a watched drive in Roon. If you want to also use an external USB drive as a watched drive in Roon, you can do that also. I don’t think you want the same music on more than one watched drive, but I’m not certain about that.