Mirror a repository from github and add a developmment branch

I am not very familiar with git, and my question might just be stupid. I would like to mirror a repository from github on my gitea server, and then add an additional branch to this repo. The goal is to have:

github
    |_ repo1
        |_ b1
        |_ b2

gitea
    |_ repo1 (mirror)
        |_ b1
        |_ b2
        |_ b3 created from commit X in b2

If I am correct, a fork is a copy of a repo at a given instant. The fork is not updated with the following commits applied to the original repo.

In gitea, I can mirror a repo. It will automatically pull new commits applied to the original repo. However, it doesn't let me push an additional branch to the mirror repo.

I would like the branch b1 and b2 to keep being updated while I work on the branch b3. Sometimes, I would merge either b1 or b2 into b3 to add the new updates which were applied.

How can I achieve this?

Creation of the mirror repo:

1. New migration

2. Check the mirror box

3. Done. The repo is now a mirror. It will update automatically.