How do I add special/bonus episodes to a series?

Hi malayali-

For these kind of episodes, in general: 

1.  The episode(s) can be added without an episode season/number and will be listed under the Unknown section.
2. The episode is listed as episode number 0 of the season that it belongs too.

For a more clear example, you can check the following Doctor Who title where the Christmas specials are listed as episode 0 of the respective seasons and the 4 special episodes released were added to the Unknown section. 

You can also add keywords to them, for example.

Let us know if you have further questions!

Cheers!