We have an existing /dev/sda disk, we are adding an identical /dev/sdb
I want to place these two disks in a raid-0 array.
Can I do that without reformating /dev/sda?
No. But you can create a RAID-0 array containing only /dev/sdb, copy the data from /dev/sda to /dev/md0 , then add /dev/sda to the array. Voila, data preserved!