It is easier to answer these sort of questions when posted to linux-raid@vger.kernel.org.
However you just need mdadm-3.1.1 or newer, linux kernel 2.6.32 or newer and then
mdadm --grow /dev/mdXX --level raid5 --backup /some/file/not/on/the/array
It will take a while rearranging all the data, but when it is done you should have a RAID5, and while it is going you should still have access to your data.
