How do I adjust the composition of a gas system in SimScape?

Hello guys, I am currently trying to model a gas reactor in SimScape. My goal is to have two different reactant streams and one product stream. I already know what the feed conversion of the reaction is and what the compositions of the reactant and the product streams should look like. But I need the adjustment for a pressure simulation.

In other words I am trying to change the composition and thereby the properties of the gas within the model. Is this possible with SimScape?

I already found the 'Gas Properties (G)' Block, but this only applies for the whole gas system and not only for a single stream.

Thank you for your help!