Modifying existing Addon?

I have read up about creating your own addon.
In Eclipse i mannaged to create a new addon using the wizzard.

I am wondering however if it is possible to download an addon and include it into a project
so it can be changed and recompiled? Is there somewhere an tutorial how to do this?
as i have no idea how to acomplish this.
For example an addon has the folders/maps/directories client, ui and util i have no idea at all where to put them.

thanks in advange