Actually there is a section in the plugin settings where you can modify the labels for the different data fields (see screenshot). In the example I changed the “Fax” label to “Appointment”.
Then in the frontend, whatever you type in that field in the stores will appear with that label, but honestly I don’t like this approach too much even if it is a bit simpler. Adding custom metadata fields is not hard and it is “cleaner”, but it is up to you 🙂
Oh thanks, I must have been blind! That solves it. 🙂