There are some great "Retro-Fit" zoning systems on the market today.
I agree with Dave that you need to contact an experienced retro-fit zoning contractor (or two) in your area that can evaluate your system.
What we have found over the past decade is that zoning an existing (ineffective) duct system can overcome many duct inadequaces and that the ducts feeding those zone will handle up to 200% more air than what they were designed for.
If you need names of any local contractors using the Arzel zone system let me know.
Good Luck!