Hi Mark,
The exporting/importing functionality is indeed not documented yet. It is basically a back-up service. Export -> Import should be used within the same domain/subdomain. Mixing instances within separate installations may not work, due to the differences in some object IDs (posts, taxonomies etc..).
If the offline site is an identical copy (base URL, posts, categories, users etc..) of the production, then it should work. If not, then probably replicating the settings manually is a better idea. If you have a specific theme/coloring setup, you can export that individually as well, that works regardless of the installation differences: https://i.imgur.com/noRHz4Q.png
