How, using only push-ups, can I best add pectoral m and definition?

I don't think you can attain your goal of adding pectoral m and definition using only push ups. Since you are already working out with weights, why not make the best of it? Using free weights is the best system of building up your muscles. There are, however, different weightlifting programs for building muscle m, getting muscle definition, and building resistance and stamina. Decide what you want to do with your body and get the training instructor to help you with the right programs. Body building is a science which follows the laws of logic.