You can extend your internship while in the U.S., even if you have, in the meantime, obtained your diploma or no longer have a student status.
An internship extension is an extension of the INTERNSHIP USA program that begins the day after the end of your initial internship. It allows you to complement your internship with an additional period within the same host company.
You can apply for up to two extensions of the same internship as long as the total duration does not exceed 12 months.