There's no dedicated feature in CDpedia at the moment that does the import for you automatically but I know that several users are keeping track of that information in iTunes by creating a smart collection with the criteria "Date added is after [today]". That'll then keep adding all new entries to that collection as you add entries in iTunes and when you're ready to do another import into CDpedia, you only have to import that smart collection. Once that's done, remember to change the date on the smart collection to the current date so that the collection will keep updating itself with newly added songs/albums.
If you've already done one import into CDpedia and don't remember when this was, you can use the 'Date added' column in CDpedia to see what the date added is for one of the entries you imported from iTunes last time. (All imported entries are marked with a blue note.)