From what I know, the zaggfolio is designed to hold the iPad (and only the iPad) snugly. Even screen protectors have the risk of being dislodged. I doubt it can accommodate any back or cover, however thin.
Your best bet may be an external Bluetooth keyboard lie the apple wireless one, with a case like the origami workstation or the logitech folding keyboard, which is designed to work with a smart cover, but appears to be ultra bulky.
As for whether It can fit in the slot, I doubt it. Even if it can, it will likely not be very stable. Else, zagg wouldn't need to redesign their products to cater for the new iPad.