We are glad you like ATUM. Thank you!
Regarding your export question: indeed, you can export as many meta keys as you want (known or unknown by ATUM) from orders or from any other post type that we currently support. Just note that, as the export uses complex SQL queries to prepare the data to be exported, and every meta key you add requires an extra table join in the SQL query, there is a limit in mySQL of 61 joins and, if reached, could fail. We are actually thinking about how to bypass this issue. But, for the time being, you could create separated templates and check some meta keys on each of them.
And FYI, we are going to launch the Import module's beta version today. If you want to participate in the beta testing for the Import, just come back to this forum later and we'll post a sticky thread where we'll explain what to do