This is a two part question.
1) Can you reshape raid 5 to 6 with mdadm on linux (fairly sure you can, but cant get mdadm to accept it)
2) Can you do 1) while also growing the array in one go ?
Yes, you can reshape raid5 to raid6, provided you have a recent enough version of
mdadm. In his blog, Neil Brown, the lead architect and developer of
mdadm's current capabilities to reshape arrays and change raid levels.
The key points relevant here are as follows:
You need version 3.1 (or newer) of
mdadm, as well as a fairly recent kernel (at least 2.6.31).
A 3-or-more drive raid5 can be converted in-place to a raid6. The following command should do it:
mdadm --grow /dev/md0 --level=6
mdadm --grow /dev/md0 --level=6 --raid-disk=6