I'm not really sure what you are asking, but this is my interpretation:
1. OS on SSD - answer is YES
2. Existing RAID0 to RAID0+1
For question 2, I believe the answer is NO. You will need to backup up all data off the existing RAID0, then break the RAID, reformat the used drives, then rebuild the RAID, and restore the backed-up data back to your new RAID.