So I have a bit of a weird speedrun which I would like to keep a secret, but it is not a game. Can I still make a category for it (bc there is not one for it) and submit it? To say the least it is not a game, nor IRL, I think it would be pretty funny to do a run of it. So my question is, is this possible and not against the rules?

If you wanna submit a game, then you can submit a support ticket (page found from Help > New ticket), to maybe get it accepted or rejected. It'd be under the "Add a new game to Speedrun" option.

The rules for requesting a game are here: https://www.speedrun.com/support/learn/requesting-a-game

There's also some on the actual support ticket page too.