Due to the highly restrictive criteria of thetvdb.com not allowing shows to be added that have not yet announced an air date, this means they also cannot be added to Sonarr. Often these series are already showing up on websites like IMDB (which, I know, has less accurate episodic information for TV shows but seems to have more acceptability on including TV Shows in their database that are still in production.)
One of the biggest advantages to using a tool like Sonarr is that when you discover a new TV Series that’s being developed, you should be able to add that show to your Series list and once the show has an air date and more details, it will automatically start downloading.
Without this, you need to remember, or keep separate lists of shows you’re interested in, and then wait for them to appear in thetvdb before you can add them to Sonarr.
Perhaps a feature could be added to add TV shows by fuzzy name: allowing you to keep an unmatched series in your list, allowing users to perform searches against them later which may match them up against series that have eventually wound up in TheTVDB.
Alternatively it could accept adding of series by IMDB number, thereby allowing the series to exist in your Sonarr installation but metadata wouldn’t be collected until that IMDB number matched up with something from TheTVDB.
(This is more a frustration of the limitations brought on this community by TheTVDB’s policies for adding shows, but I feel that more could be done within Sonarr to alleviate this pain by giving users more power over adding shows to their series list. I know this feature has been requested before and I am not the only one frustrated by these limitations. I hope there can be some nice middle ground found here.)