How do I add a "Best of" DVD to an existing TV series

In essence, we are talking about “local extras” here.

If your library is still using the old Python based metadata agent, all you can do is take a look into TheTVDB, the “Specials” season and see if any of your clips are appearing in there. Then name the files as if they were regular episodes, but with season number 00 and the episode number from TheTVDB list.
If your specials are not listed in TheTVDB, make up episode numbers for them. It is best to pick numbers high above the last listed special on TheTVDB.

If your library is already using the new tv show metadata agent, you can add specials at the show, season, and episode level.
Be advised that episode and season-level extras currently won’t show up, except in the Android mobile and iOS Plex clients.
see [BETA] New Plex TV Series Scanner for details.