I just completed all of the Starcraft 2 campaigns from 0 to 100% achievements in 25 hours and 3 minutes which I believe is a world record, surpassing GiantGrantGames from 2 years ago by 29 minutes. His stream joked about adding it as a speed run on here, but since they break each Starcraft campaign down as their own thing I don't see anyway to add it.

I have tried to add a new series, but it looks like I would have to be a mod of one of the games, which I am not. Are there no categories of multiple games being played consecutively?

You would have to reach out and ask a moderator of one of the games or a game and the series if they'd consider adding a category for it. Don't request adding a new series, reach out to one of the current series mods about it - in some cases, they may be willing to create a board for 'Multiple Starcraft Games' if there's a reasonable call for it in the community and it's approved by site staff, or to have a general category extensions page for the series, that's been done with stuff like Fallout and Ratchet and Clank off the top of my head.