I'm looking for a Wordpress Plugin Expert to build a custom Plugin

I'm using a purchased WordPress theme that's updated often. The problem when it updates, it obviously removes all the custom changes I've made on the theme. What I'm attempting to do is create a plugin that allows itself to be injected into a WordPress theme where I can create custom pages/functions that interact as if they're part of the theme. I'm a developer, but my experience is super limited with WordPress plugin API. Please put up a bid, and explain in your comments how you would accomplish this task. Thank you