You have several options, you can give up the fan control output. Or, to keep the fan control we're trying the bypass line instead. The bypass line is used to signal the coil module to switch from bypass mode timing (bypass line low) to est mode timing (bypass line driven high by the ecu) when the engine starts. If you just put the equivalent bypass (cranking) mode timing into the timing map then the bypass function isn't really needed in this case with the FAST ecu, therefore you can just hardwire the bypass line high externally with a strong pullup and thus free up that FAST output for changing to nitrous control. FAST will have to rework the ecu to make this happen; you'll have to hardwire the byass input to the coil module. Hope that's specific enough...
Edit- it looks like you don't have a Buick v6 though, so never mind...
TurboTR