Doesn't mdadm do this already?

mdadm seems to hot add things in for me on raid1:

mythtv test $ mdadm --create -n 2 -l 1 /dev/md2 /dev/loop1 missing
mdadm: array /dev/md2 started.
mythtv test $ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 loop1[0]
102336 blocks [2/1] [U_]

unused devices:
mythtv test $ mdadm --add /dev/md2 /dev/loop2
mdadm: added /dev/loop2
mythtv test $ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 loop2[1] loop1[0]
102336 blocks [2/2] [UU]

unused devices:
mythtv test $

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