Applying Momentum source to a cell adjacent to supply diffuser
 

I have been trying to implement the momentum method to model a supply diffuser. In this method, we consider the actual area of the diffuser and decouple the mass and momentum boundary conditions. However, this is not possible using star ccm+ or any commerical CFD packages. As a result, The inlet mass flow rate is given at the grille diffuser surface, and the momentum flow flux provided by the diffuser is specified as a momentum source added to a volume (the first gird layer) adjacent to the diffuser.

I am not sure how to add a momentum source to a cell, I was wondering If any of you do and If you could please explain this to me. I would greatly appreciate your expertise.