If you are using LVM you can use the "pvmove" command.
I haven't tested this, so the first thing you almost certainly want to do is to set up a test system that is like your system is set up now, and then go through your planned procedure to verify that it works as you'd like and flesh out the details.
It would look something like this:
- Create the RAID array as normal.
- Use "pvcreate" to turn it into a physical volume.
- Use pvmove to move the LVM to the RAID array. (Fully read the pvmove man page to understand its limitations)
- Remove the non-RAID pv from the LV.
- Move /boot over to the new drives (probably via another RAID-1 array)
- Update the boot loader config and put the boot loader on both drives.
- Make sure the initrd has the appropriate RAID modules.
- Reboot and test.
Note: there is an issue with pvmove that may cause it to deadlock when doing pvmove on the root partition.
Give that a shot to see if it works.