Add in-app billing to an existing Android App

Hi, We have an existing Android application that is fully functional; however, we need to add in-app billing feature to the app so that it can do monthly subscriptions. You should already know how to code in-app billing for an android app. We have the full source code for the application. It is not in Google Play at this time. We have a Google Play account and you will write the necessary code to accommodate our feature request. Thanks.