So it appears that nothing like the drive extender that WHS has, exists for linux. Discussion on AVSForums leads me to believe that.
Explanation on how WHS Drive Extender works.

So I have decided to write some code to accomplish something similar.
What are the needs:
– amalgamate a pool of partitions/folders and make them seem like one folder.
– when data is written to the pool, the software manages where the data is stored (what partition/folder in the pool)

Since everything is running from partitions/folders, FlexRAID can be applied to the same folders, to add a level of redundancy to the whole system (something WHS doesn’t have to my knowledge)