Setting PythonSdkAdditionalData user paths.

I'm about to write a plugin for PyCharm that creates a new SDK configuration for each project.
The configuration comes from an external source.
But i'm having trouble adding paths to a configured interpreter.

I can see the paths by doing: ((PythonSdkAdditionalData) sdk.getSdkAdditionalData()).getAddedPaths()

But how do add paths to the configuration?

Thanks in advance.

/Fredrik