Hello...I am decorating cupcakes tomorrow and I have some leftover Italian meringue buttercream that I will be using. IF I don't have quite enough, I'm wondering if I can just mix in a bit of whipped store bought icing to make up the difference. I think I'm just worried that mixing these two might change the consistency and I'll ruin my buttercream. 

I have no idea.  Try mixing a spoonful of each together and see what happens.