You have to start the import from your own code (that is, create your own Import Data page) and set IsCopyByReference to true on the DataImporter object.
This will keep the PageGuid during import instead of creating new identities every time you run the import.
I suggest that you reverse engineer EPiServer.UI.Admin.ImportData with Roeders Reflector.
It would still be nice to have the option in the UI instead of having to create my own import data page.
I would like the ability to add a language to an existing page using the export/import feature.
For example, right now we have a full website in US English on a production environment. We have a staging environment that mirrors the production one. If we were to add a Spain Spanish language to the pages and completely translate the website to spanish, then exporting and importing those pages will bring in all NEW pages to the website instead of just adding the new language pages to the existing pages.
The way it works now is not ideal for adding a new language to the existing pages using a staging and production env to push/deploy content.
Unless I am missing something? I have tried several options in the export and it always creates NEW and does not replace/add to existing pages.