Sorry, I have to tell you the truth that it is impossible to use JBOD to concatenate disks without erasing data. To set up the concatenated disks, you are required to format and erase both hard drives first. Or else, you can't concatenate them.
As you've already discovered, the function of creating concatenated disks is to expand the storage capacity of the volume, but in most cases, it is not worth doing that.
On the one hand, this will erase the disks. On the other hand, combining individual external hard drives into one is open to a huge point of failure. That's because, during the process of setting up the two drives, a power outage, disconnection of one of the disks, or the Mac asleep will interrupt the disk combination procedure. If you put the two disks into a dedicated external enclosure for the JBOD, these failure situations will reduce a lot.
It is not recommended to add storage space to the hard disk by creating a concatenated drive but clean up your hard disk in routine. You can organize your files on your disk by removing duplicate files, deleting junk files, tidying up outdated data, etc. Otherwise, no matter how much extra storage you add, the accumulated files will also run out of the available storage space. Maybe you can try iBoysoft DiskGeeker, a disk management tool, that can help you manage disk data, clone disks, test disk speed, check disk errors, etc.