Existing AFPS Time Machine, adding identical drive to create RAID1, possible to do without reformatting?

I did a lot of research already and it seems like this was definitely possible before AFPS (see http://hints.macworld.com/article.php?story=20041013121106176) . The command diskutil appleRAID enable mirror /dev/diskXsX could be used to group an existing drive into a RAID1, and then you could add the new drive and 'rebuild' the second drive. However, I'm trying this with an AFPS Time Machine drive, and I'm getting Error: -69819: File system resize support required, such as HFS+ with Journaling enabled. Any advice? I don't have another 6TB drive to temporarily offload the Time Machine data to, so I need to figure this out if it's possible.