Mini-Mac Server as an NAS

I have been wanting to start digitalizing my music and placing it on a server.
I have a Mini Mac Server that I have been using for my business that I am replacing with a Windows Server (the programs we run are Windows based and the problems that we have had with the software to convert Macs to virtual Windows Machines have been so problematic that that they have almost closed us down). I was wondering about using the Mini Mac as a home NAS, since I would like to get some use out of it and since I otherwise have a Mac based system at home.
My main concern is this: I need about 4TB, and the Mini mac Server has 1 TB, configured as 2 500 MB drives in a RAID config. Can I add extra drives
and have them work seamlessly?

I would start with a new external array rather than add to the internals. 2TB external drives are now less than $100. I would get a couple of those for data storage and a few more for backup. A program like Super Duper can manage the backups.

I have three 1.5TB drives in a concatenated array so it looks to the OS like a single 4.5 TB drive. The advantage is it is easier to manage the library but if any drive fails you lose everything on all discs. Of course you will have a backup or 2 in case that happens.

Supposedly you can add drives to a concatenated array but the one time I tried it it didn't work properly. The drive got added but it didn't add the extra capacity. I could go to disk utilities and see that the array had 3 drives but I had three 1.5 TB drives in an array with only 3 TB of space. You can't remove a drive so I ended up having to reformat those drives as a new array and restore from backup.

No, do not make a RAID with mixed internal and external drives. It won't work. Software RAID requires all drives in the RAID set to start up within a short period of time which is very difficult to do with external drives. As soon as one external drive didn't power up in time, that drive would be considered damaged and the RAID set would be in degraded state. I have tried it before. It is very massy. Don't do it.
4TB: Wow, you got quite a music collection! Let us know how long it takes to rip that much music.

I have a couple of thousand CDs. Some of these are SACD and DVD-A and I won't be ripping these but I would like to get rid of the bulk of my collection so that I don't feel like I am living in a CD warehouse. I figure 4 TB will handle the collection and leave enough room for new acquisitions.
I kind of expect that the ripping process will take a few years, during which time I can reconnect with a lot of music that I haven't heard in a while.
What is "concancinated"?
You can rip DVD-A discs and get the hi-rez PCM. Also, when Herman mentioned concatinated he's probably using software that acts as a "volume manager". Multiple drives can be used to present one sum-of-the-parts big drive to the user. And usually you can add drives to make the volume bigger over time.

If you plan to make a big volume of some sort and it will be over 2.2TB, make sure your O/S can handle it. I would think current Apple O/S can do it and Windows 7 can do it (and linux). And also be careful that there are new disks that are being sold now that use 4K physical sector sizes and use software in the drive to do 512 byte sector emulation for the O/S. Stay away from them. And I believe there are some type of "hybrid" drives out that have 4K sectors and somewhat large solid state caches to help overcome the performance degradation of 512 byte emulation when using physical 4K sectors. Stay away from them also.