Expanding Flash Storage with SDXC I see on a Macbook Pro, you can expand your current storage space with SDHC or SDXC. Being a Linux user I know you can use RAID to expand storage with 2 disk, so storage looks like 1 HD. However, most of the time this setup happens when NO O/S is currently installed on 1 of the 2 storage units.

Would is be possible to expand my storage from the current internal flash storage that host the O/S to a SDXC card, so the storage virtually looks like 1 hard-drive from the mac os desktop?

This should be resolved software wise, however I do not think Disk Utility can accomplish or this can be naturally accomplish with out erasing the OS 1st(Which I do not want to do).
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.