How to add programmatically a source directory to a project

It is easy to recover programmatically the list of source directories
of a given project through org.netbeans.api.project with the next call:
ProjectUtils.getSources(project);.

But I didn't find an API that enable to add a source directory to a
project. In my case, it will always be an ant-based or maven-based J2SE
project.

Is there an API for doing such a thing or is there a turnaround?

Thanks for your help.