Glad that you like the addon and also, thank you for the great rating.
Regarding the ability to export and import saved sessions, theoretically it's possible and I might add it in the feature but because the WebExtensions is still limited and I can't truly import bookmarks with it may create lots of problems for anyone using Firefox Sync.
That being said, somehow you can still export all your bookmarks right now and those do contain the saved sessions but also all your other bookmarks. I'll see what I can do regarding this feature. I might find some solution after all, maybe by using the Bookmarks Import feature and not something through the addon API it might work.