Hi RolandD, it was this that made me think it wasn't possible:
https://rolandd.com/documentation/ro-csvi/migrate-joomla-data-using-ro-csvi-migration-templates
"Keeping the database ID
The migration templates have been made with the idea that you want to keep your database IDs the same. This is a big help in maintaining the relations between different parts of your website. Think about the menu items linking to articles or categories, they are connected by the database ID of the given category or article. Modules are linked to menu items by their ID, users are linked to articles by their ID. The list goes on and on. To prevent that you have to manually change all these settings, which would defeat the purpose of the migration, we have added the option to keep the existing IDs, this option is enabled on all migration templates.
One thing you really need to be aware of is that the website where you import the data does not have any of the existing IDs from the old site. If you do have them, they will be overwritten and things can look quite unexpected. So check if you do not have any overlapping IDs between your source site, where the data is exported from, and your target site, where the date is imported into. If you have any overlapping IDs, you have to decide whether you run the migration into a clean site or cleanup the site so there are no overlapping IDs anymore."