When I bought my Zen 7x7 I was tossing up between a CNC Mill and a 3D Printer and this new add-on is an intriguing idea. I will share what I have learnt so far and hopefully someone else will chime in if I have anything wrong. First to your questions:
1) You really need additional controls for the extruder. This is because a the extruder typically needs temperature control and
feedrate control - not just an spindle on/off control that is included in some motor driver boards (the Zen driver board I have does not even have this on/off but there was a suggestion on another topic of a newer board that may have spindle control again but I am there is not just some confusion). Anyway so the short answer is you really need somthing more like a 4D driver board, a 3D printer driver board (that you swap in) or a secondary controller board via USB and serial to just control the extruder and use you Zen driver board for the X,Y and Z.
Still with me ...?
2) The official answer is no ... the real answer is it might be possible with a few hacks and using a secondary USB or serial controller.
3) Generally this is not recommended as these devices do not do a good job of providing a good continuous flow of information for the driver board. There are specially designed USB to parallel adapters which will work with Mach3 and these are the only ones you can trust to work. Generally a PCI parallel port is a better option or an old refurb PC which can be just as cheap.
The real issue with 3D printing is going to be the speed of the Zen 7x7. With the common default settings our 7x7 can feed reliably at about 300mm/min but 3D printers like the makerbot do 3000mm/min - ten times faster! The new ultimaker manages reliable printing up to 300mm/min (claimed) - 60 times faster!!
The issues with this speed difference is not just the extra wait for prints but also the issues with the flow and hardening rate between layers which can affect the strenght, structure and shape of your final model.
To start to get your head around the conversion possibilities and issues check out these pages which are some ideas of working through the issues we face in a conversion:
EMC RapRap Wiki Page:http://objects.reprap.org/wiki/EMCRepRap
Thingverse page on 3D printing with Mach3http://www.thingiverse.com/thing:5233
I'll post anything else useful I find.