I cannot think of a way to do this. If the brace prevents the beam from deflecting vertically, then it doesn't care whether the deflection comes from gravity load or lateral load. Isn't that what you want the analysis to tell you? How the structure is actually behaving under the applied load. Perhaps rather than adjusting your model, you need to beef up those columns so that they can take the gravity load.

Could this be a case where the gravity loads are already present in your structure (with the beam deflected as a simply supported beam). And, you are just adding the knee braces to the existing structure.... Sort of a staged construction. Where the gravity loads act on the un-reinforced frame. And, any additional load (i.e. lateral load) would attributed to the reinforced frame. That's a non-linear type of analysis that RISA doesn't do automatically. However, you can manage it by manually combining the results from different models.

For example, you could eliminate the gravity load from the beam and just attribute it directly to the column. Then run the beam design in a separate analysis and use superposition to combine the beam results together.