Unfortunately we don't have a built-in document type for this yet.
The best way to implement this would be to create a new subclass of AbstractPluginDocument that stores the data with an associated exporter and/or viewer for getting the data out. However, this is a lot of work so your best bet is probably to save a csv file straight to disk outside of Geneious.
We are working on a new document type that can be used to store, view and export text or html which would suit your needs and save you a lot of work. I've put in a feature request and you can update your plugin to make use of the new document type when it's available.