Turning 2disc JBOD volume into RAID

You cannot convert two independent volumes into a single RAID and retain data from both.

There are a number of ways you can go about creating a larger RAID by adding drives.  For example, add an 8TB drive to the existing primary JBOD (the one contaiing the home folders and apps) to make it RAID1.  When that sync completes, swap the other drive in that group with another 8TB and expand it.  You now have an 8TB RAID with the content of the original primary 3TB and enough space to move the content of the other 3TB JBOB.  After transferring the data, you can destroy the 3TB JBOD and remove it, after which you can reformat it and add it back to the 8TB volume if you so desire.

Note that adding a 3TB drive to a RAID consisting of two 8TB drives will not work in many cases.  But in your case, there will be a 3TB layer and a 5TB layer of the RAID, so you can add a 3TB to that initial 3TB layer.  That layer will become RAID5 and the 5TB layer will remain RAID1.

This will work with any size and number of new drives where the resulting RAID will be at least large enough for the content of the two original JBOD volumes.  Adding two 3TB to the original primary volume is, therefore, another option.

If you are having trouble following this, tell us what size drive(s) you intend to add and I can give a little more detail.