Currently the main branch repo contains instructions on how to add a challenge. It appears it is not giving advice on how to create an extension but instead, the prior challenge type.
https://github.com/scaffold-eth/se-2-challenges?tab=readme-ov-file#-contributing-guide-and-hints-to-create-new-challenges
We are going through these steps currently so we can submit a PR to change this if it seems helpful.