expansion unit questions

I'm playing with the idea of using a TVS-1271U-RP + REXP-1600U-RP to store some data (or some other similar configuration), but i'm curious about how the storage configuration would work.

I'm quite sure that you cannot do raid between the disks on the nas + the ones on the expansion unit, so that means i'd have to create some sort of raid volume on the nas + a raid volume on the expansion, and combine it into one single storage pool?

If that is the case, is this storage pool essentially a raid-0 of the two raid volumes, or is it just a software based pool?

Thanks for the info.