I've been feeding filament from an external spool on my DaVinci 1.0 using my Big Spool Holder (http://www.thingiverse.com/thing:583800), which was derived from Ranh316's DaVinci Rear Filament Spool Hanger (http://www.thingiverse.com/thing:409100).
Works OK but I don't like the built in filament guide: It crimps the filament if you open the top too far and makes changing filament tricky. I tried passing the filament directly through the rear slot, but it tends to pull the filament off the side of the spool, eventually resulting in a jam.
So I made this filament guide, which clips to the rear slot and keeps the filament feeding properly. It doesn't require any modifications to the printer.

I printed it with XYZWare's Good setting.
Mount it in the slot as seen in the photo. Filament runs from the spool through the guide, rear slot, and the plastic tube, then to the extruder.