All the programming in the NAE is viewed through internet exploded. I'm not exactly familiar with the Z series with the lon, but I would assume that all they are doing is reading data and commanding some setpoints. I would highly doubt with all the OEM equipment I've dealt with in the past that you will be able to control the VFD output once you do get into the NAE.
I know in the past what I've done on your type of situation is use my own DP sensor and sent my own signal to the RTU controller. Your factory transducers will be your run of the mill "economically priced" transducer that we all can buy from Kele, ACI, etc. They should have a 4-20ma, 0-5vdc or 0-10vdc signal going back to the carrier control board. What I would do is take the factory transducer out of the unit, install it in your common duct work and run that to your own programmable controller. Then take 3 seperate AO's out of your controller back to the input on the RTU controller. You can program your controller or three controllers with logic so that all three units are getting the same exact static signal all the time. You'll have less labor doing that than beating your head against the wall trying to get the factory controller to do it.