Not a stupid question at all - but there may be several answers.
Simplest is that you can plug in a USB drive (thumb drive / USB hard disk or whatever) in the Pi, reindex MPD and Volumio will be able to access and play that music once MPD has completed it’s indexing (which may take some time). This I do.
If you also want to share that music out to other network attached devices then this may be a little more complicated. Certainly it should be possible to create a CIFS/SAMBA share form the hard disk enabled Pi and then have other volume clients access this CIFS share, indexing the music on each Pi via MPD. This I have not tried.
I am not sure (today) if you could create a NAS like UPnP visible solution to the rest of the network within the same Pi but it’s only software so technically it’s possible.
Hosting 24/192 audio on a NAS and having multiple targets across the network read and consume that media will certainly increase CPU overhead which if the local “NAS” Pi is also playing such high rate media may reduce available CPU for playback, potentially causing gaps in music and stuttering. Basic mp3 should I guess be fine.
Hope one of the above helps in some way.