Expanding Flash Storage with SDXC
The only viable options for RAID using just 2 drives would be RAID 0 or 1. 

RAID 1 wouldn't increase your storage, so 0 is the only remaining choice. 

Even if you could RAID an internal fixed drive to a removable SD card (&amp; without first formatting both), the drive would only be 'whole' so long as both were present (&amp; still functional.)
If either drive was missing (or broken) the entire RAID would be irrecoverable.

Trusting an entire RAID to the vagaries of an SD card would be playing russian roulette with your data.