I have the following.

    Base Repo
    - main

    MyFork Repo (from Base Repo)
    - main
    - feature A
    - feature B

    AnotherFork Repo (from Base Repo)
    - main
    - feature C

I have forked MyFork Repo from Base Repo. That works fine. Now I'm interesting in the feature C branch from AnotherFork Repo. I can't fork AnotherFork Repo because I already forked Base Repo.

I also can't just clone AnotherFork Repo and create branches there or push to feature C. That is just with PRs possible.

Can I do then i.e. something like that?

    MyFork Repo
    - main
    - feature A (is linked to Base Repo)
    - feature B (is linked to Base Repo)
    - feature C (is linked to AnotherFork Repo)

Or do I have to delete MyFork Repo and create then a fork to AnotherFork Repo?

There are two typical workflows: central and distributed.

Central repository

In this workflow, there is a central / "official" repository from all other pull and push. In this workflow, you'd push feature/C from AnotherFork to Base and then pull feature/C from Base to MyFork.

$ cd AnotherFork
$ git push feature/C

$ cd MyFork
$ git fetch && git switch feature/C

Distributed repositories

In this workflow, some repositories know others as remotes, and you can directly pull feature/C from AnotherFork to MyFork.

$ cd MyFork
$ git remote add AnotherFork proto://url/AnotherFork
$ git fetch AnotherFork
$ git switch remotes/AnotherFork/feature/C

Additional reading

• man git-remote
• man git-fetch
• man git-branch
• 2.5 Git Basics - Working with Remotes
• 5.1 Distributed Git - Distributed Workflows