This can be done in NX Advance Thermal. Supposing that the valve is controlled from the temperature at some location I can see 2 ways of doing this 1) I think that this could be approximated by changing the flow in each of 2 branches vs temperature. This could be done with a simple include file that would control the flow resistance in each brach versus temperature - increasing the flow resistance in one branch while it decreases in the other. An example of an include file defining a table of the flow resistance versus temperature is below:
TABTYPE 8 FLOWRES TEMP
TABDATA 8 0.05 0.0
TABDATA 8 0.07 200.0
PROP 14 FLOWRES 1.0
INTERP 497 25 8 1.0
The fifth field in the PROP card should be blank (reserved for table).
Head loss for FLOWRES element 497 is interpolated from table 8 using the temperature of element 25.
The other option, if the above does not work for you would be to create a user written subroutine. You can write fortran code that is executed at run time and do very complex functions such as controlling the flow in your duct network based on your own functions.