External, no. At least I can't think of a way to make that work that way.
As for changing the media, you just pull the reaction chamber out (it comes with a cap to keep it from leaking on your floor while taking it to where ever you change your media). Dump out the media, insert new, run RO/DI through it (it'll run out the bottom of the chamber). Then re-insert it back in the outer housing. That's it.
No messing with special wrenches to take it apart, cartridges, filters, etc.
It really is silly simple.......which makes it perfect for me.
They've got a couple of good videos on their site that kind of shows you how to set it up and how it runs if you want to check it out.