Q. I already use an App (My Fitness Pal, Lifesum etc) to log macro nutrients and/or meal composition

A. So long as your App can synchronize its data with the Health App and your meal logging includes a “Dietary Energy” component absolutely (Hint: both of these conditions are highly likely). Just make sure in MyFast Track settings that you have “Any Source” selected under the “Track meals/energy from:” option.