I think all this is doable, but it's tricky error-prone stuff (at every level, software, hardware, firmware, wetware and slippery finger ware, etc...) so it's best to have a reliable external backup of at least the critical stuff before even trying it, esp. for the first times.
RAID 1 is the simplest and easiest to maintain RAID level, and yes, for the most part drives can go back and forth from being RAID 1 drives and simple non-RAID'd drives, but it's still easy to mess up and lose all your stuff. For this, it's generally better to emphasize an external backup first before getting into RAID, esp. on a non-empty data set.
The way to do single drive to RAID array conversion in nVIDIA RAID configurations can sometimes be tricky. There are also several nVIDIA RAID revisions out there, so I can't say that what I've seen in the past will for sure apply to yours. But, there's a good chance that it will.
In order to install the OS on an nVIDIA RAID array, you must install the RAID drivers during the OS installation. This generally means creating driver diskettes with the RAID array, of course having access to a floppy for this purpose, and pressing F6 during the OS installation, and installing two drivers (SATA/IDE and RAID) during the OS installation.
Moreover, getting the nVIDIA RAID array to run with a single drive can mean running in a very non-intuitive configuration -- single drive RAID 0. This is in effect a regular drive, but with the RAID software installed. And once the drive is running with the RAID software, it's easier to convert that drive from one RAID format to another.
In converting a single RAID drive to a RAID 1 pair, you sometimes need to to tell the system to copy the data from the primary (original) to the secondary drive. The system is dumb, and if you make the mistake of specifying the wrong drive here, you can wipe your good data with an empty drive. The terminology isn't clear either, but as I recall, you issue the "rebuild" command to the drive you want rebuilt, not the drive from which you want the data.
I could be wrong about some of this due to revisions in the RAID hardware or software, but that's what I recall -- you must start off with a RAID array, and this might mean the non-intuitive single drive RAID 0 configuration, and from that you can create the normal RAID 1 configuration.
RAID 1 installation should really be more straightforward and intuitive, and sometimes it is, but I recall having to go through this non-intuitive part at one point, which is why I'm mentioning it. It could be that I was trying something a little trickier, such as converting in-place to RAID after having installed it without the RAID drivers provided via floppy during OS installation.
Again, for all of this stuff, you have significant data risk, so starting off with a full backup that you put aside and don't write to any further would be a good idea, unless you can afford to lose everything. A backup is a good idea in any case -- even with a functioning RAID 1 setup, an external backup will help protect you failure modes that the RAID 1 setup can't -- e.g. malware wiping the data. A backup can also help protect you from any mistakes or significant omissions I have made in this post.
The data backup is also a good answer to data migration and expansion questions. If you want to move / expand / etc., then make a full backup, create the new array, and then restore the desired data from the backup.