Can I add cocoa to chocolate to approximate a higher % of cacao?
Sure, you can add cocoa - but think of it as adding it to the brownies, not to the chocolate. And the thing to worry about isn't extra weight, but rather whether you're adding enough dry cocoa to upset the dry/water balance in the recipe. With only 10g and something flexible like brownies, it should be fine, but you'd have to be more careful with larger amounts or more sensitive baked goods like cookies.