Translations:FAQ/83/en

Yes, but it is not trivial, nor is it recommended. Since the method to do this requires a good understanding of Portage internals and commands, it is instead recommended that you patch the ebuild to do whatever it is that you want and place it in a Portage overlay (that's why overlays exist). This is much better for maintainability, and usually easier. See the Ebuild HOWTO for more information.