Is it allowed to add new content in the projects to make it richer, besides from the user stories?
Absolutely. You can add other content and functionality just as long as you meet the minimum user story requirements. In fact, sometimes the user stories can limit what you would really like to do. In those cases, you could make one version for the certification and then an improved version (which may not meet all of the FCC user stories), but looks and works exactly how you want. The latter version is what you would probably end up putting on your portfolio.
Of course allowed. Without new content your project will be of little interest.