A plugin cannot create Roman Numerals or Nashville numbers, only Chord symbols.
The plugin-API allows for creating HARMONies:
harmony = newElement(Element.HARMONY);
This creates a "Chord Symbol" but not a Roman Numeral symbol or a Nashville Number symbol - you get an ordinary "Chord Symbol". The API does not expose any way to set its subtype (to RNA or NN).
Existing RNA- elements CAN be altered (change its text) and deleted - BUT not created.
It would be useful if Roman numerals and Nashville numbers had all the functionality (applicable) as Chord Symbols, including creating them from a plugin (and possibly other use cases?).