Thanks a lot. I referenced your idea and finally extend it recently.
Here is full commands for others who might need to do in the future.
Assuming /dev/sdb and /dev/sdc are existing 4TB drive, and we are going to add /dev/sdd which is 8TB drive
1. Now, mount new drive, format with gdisk, mark it as 8300 (Linux Filesytem), so we can copy from /mnt/md0 to /dev/sdd9. Edit /etc/fstab to make sure it mount /mnt/md1, options are copied from existing entry created by openmediavault previously/dev/md1 /srv/MYRAID ext4 defaults,nofail,user_xattr,noexec,usrjquota=aquota.user,grpjquota=aquota.group,jqfmt=vfsv0,acl 0 2