It's really tough to answer questions like this without knowing the entire recipe (starting proportions) and what end-effect you're trying to achieve. Just curious, why do you feel the need to add more liquid?
Not knowing the full context of your goals, my best recommendation is to test the recipe by making small controlled changes (one change at a time) until you achieve the result you want. But, as econlady said, rarely would an increase in liquid mean a one-for-one increase in dry ingredients; it's overall proportions (typically measured by weight, not volume) that are most important to preserve - provided you want a result similar to your starting recipe.