The hpacucli command-line lets you do it. (Make sure slots are correct, of course …)

Assign disk to logical drive:

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hpacucli ctrl slot=0 ld 1 add drives=allunassigned

After synchronization finished, migrate

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hpacucli ctrl slot=0 ld 1 modify raid=1

You can do it with the GUI but it’s a two step process so migrate RAID level won’t show up at first because there’s only one disk in the array. It’s a two step process with the CLI too of course … modify raid=1 won’t work if you use that command first.

This is exactly the same process as the lists for the CLI but done through the GUI :-

First add the new drive to the array then it will re-level it to a two drive RAID 0 with half the space unused.
Second migrate the RAID level.

Why they don’t allow a one-step adding of a mirror I’ll never know, it’s trivial to code and HP’s way takes much longer because there are two re-levelings involved although it does that in background so you can carry on working while it’s doing it.