Connect a single hard disk (with data) to a RAID at a later date?

I set up OMV on a Raspberry Pi and connected a 4 TB HDD via USB. Works great! Can I connect a second 4TB HDD at a later date and connect it to the "old" HDD to a RAID, so that all data remains a) preserved and b) from now on all data is so protected that a hard drive can fail quietly. Thank you for your support