This is a known workflow of separating a comp into Red, Green and Blue channels, scaling one of them and recombining the channels, wrapped into a single effect.

The process of adding chromatic aberration is pretty straight forward if you do it to a single image.

If you try adding it to a layer with an alpha channel, that you later want to composite over a background image, you'll get into some trouble.

In the tutorial you can see different approaches to the problem, and how the plugin helps you in each one of them.