Turning RN104 into an external enclosure

Just pop the drive in won't work. As said the NAS will only mount BTRFS drives automatically.
Now what you can do is to mount the drive manually via SSH and edit the samba configuration file. This would allow you to share the disk, in theory at least since I haven't test it. That said this would also brick the web interface because some functions are tied to BTRFS and the web interface tends to edit configuration files too. To make them permanent you would have to edit the fstab SQLite database I think and this is not recommended at all.

Anyway, to boot the NAS you need a spare disk first. This drive would be formatted and then the OS will boot from it. No matter what you want to do you have to get this first drive to boot (if it contains data, just do a factory default, this will erase the drive). After that the normal way would be to copy data to that disk, then erase the source disk and plug it (after disabling x-raid if you want to), then copy the second set of data to it, erase the second source disk, copy the third set of data, erase the third source disk. This is the best way for many reasons like preserving Netgear support, using BTRFS built in safeties, allowing to use raid, keeping a functional web interface, keeping additional services like ReadyCloud or dropbox sync... I know this is long but this is the best way and by far.

The other way around is to mess with SSH and you may get an unstable system, you will miss function and run in degraded mode. This would be true for any NAS as they are nothing more than automated linux, and when you send something unexpected to a computer program, it usually crashes. So you have to understand those automations and avoid to send something disturbing in those mechanisms. That said, if you do that you are dealing with a standard linux configuration, so classic samba configurations should work (provided they are not erased regularly) and since NTFS is supported by the ReadyNAS for USB, it should also be supported internally by linux (though the automations instruct to specifically not mount NTFS drives automatically for security reasons). So your job is to log in with SSH, mount the NTFS disk to a custom location, edit the samba file and reload samba. This should work until reboot or a samba reload with another configuration. The fact is I don't really know how to prevent configuration rewriting, I think this has to do with the SQLite database but that's all I know. As for the disk remount at boot, I would normally use /etc/fstab to do it, but I'm not sure that this would work on the NAS, alternatively, a script set to run at boot could mount the drives (use their unique identifiers or you will have a surprise). That's all I can tell you, you have the basic steps, but this is really not advised. I haven't tested it so, if you decide to go with that, you will take your responsibilities.
Maybe using a sharing protocol that is not handled by the NAS, this would be easier, like using vsftpd on a non-standard port. That way the configuration won't be rewritten.
Anyway, no matter what you do, SSH is the only way for it to work on the NAS unit and this requires some skills, will break the web interface and will void the warranty until a factory default.

The last way is to use USB disks but though this would let you share your disks right away with no real drawbacks, you will quickly run out of outlets, space and USB ports.

The no NAS solution would involve some kind of desktop computer (tower or rack), you put the drives in, resize one to free some space to install a windows using Gparted for example, then install windows and use windows (or linux or any OS) to share the disks. That way you have no automated process to deal with. If NAS are so automated that just to make them simpler (and enhance security by automatically monitoring and sending alerts), but if you don't want that, just use a plain old computer, this would work.