I guess the first question I'd as is why :)

What's your objective here? If it's to eliminate the requirement for a backup RAID is a terrible idea. I've seen a hardware RAID controller fail and write garbage to a big RAID10 array. Neither HP nor Ontrack could recover the data and we ended up restoring from a backup that was older than we would have liked, but that's another story (one where a sysadmin almost got fired).

Can you migrate from RAID1 to RAID10? Yes, but notice I said migrate and not convert. As we all know RAID10 is a striped set containing two or more RAID1 mirror sets. You'd have to break the RAID1 mirror and rebuild it as a RAID10 array, but your post kinda leads me to believe that you don't have a solid backup solution in place, which would make the whole project kind of a horrible idea.

disclaimer: My experience is limited to hardware RAID; I have no experience with mdadm.