Can I add a raidz vdev as a mirror of existing vdevs

I have a zpool with a "stripe of mirrors" structure:

mech
   mirror-0
     disk0
     disk1
   mirror-1
     disk2
     disk3
   ...

I want to convert it from mirrors to raidz as part of an upgrade, with 
the final form looking something like this:

mech
   raidz-0
     disk20
     disk21
     ...

I'd like to be able to attach the new raidz vdev as a mirror to the 
entire pool, have it silver, then detach the old vdevs.  Is this at all 
possible?  I'd rather not create a whole new pool and copy everything 
over because doing so would require downtime.