It is definitely possible to run multiple services on the same pi, as long as the pi is powerful enough.
You mentioned the NAS software has its own .iso - if you used that you’d then have to install magicmirror on top. You may come in to some conflicts there and have to tweak, but definitely worth a shot.
Or if you start with your magicmirror install it is fairly simple to expose a shared folder to the network and have the pi act as a NAS also.
Good luck!