Assuming you have repos `foo` and `bar` and want to move them to the new repo `mono`.

    $ ls
    foo
    bar
    
    # Prepare for import: we want to move all files into a new subdir `foo` so
    # we don't get conflicts later. This uses Zsh's extended globs. See
    # https://stackoverflow.com/questions/670460/move-all-files-except-one for
    # bash syntax.
    $ cd foo
    $ setopt extended_glob
    $ mkdir foo
    $ mv ^foo foo
    $ git add .
    $ git commit -m "Prepare foo for import"
    
    # Follow those "move to subdir" steps for `bar` as well.
    
    # Now make the final monorepo
    $ cd ..
    $ mkdir mono
    $ cd mono
    $ git init
    $ touch README.md
    $ git add README.md
    $ git commit -m "Initial commit in mono"
    
    $ git remote add foo ../foo
    $ git fetch foo
    $ git remote add bar ../bar
    $ git fetch bar
    
    # Substitute `main` for `master` or whatever branch you want to import.
    $ git merge --allow-unrelated-histories foo/main
    $ git merge --allow-unrelated-histories bar/main

    # Inspect the final history:
    $ git log --oneline --graph
    *   8aa67e5 (HEAD -> main) Import bar
    |\
    | * eec0abd (bar/main) Prepare bar for import
    | * 9741d6d More stuff in bar
    | * 634ba3d Initial commit bar
    *   43be6e9 Import foo
    |\
    | * d4805a0 (foo/main) Prepare foo for import
    | * 4d2ca10 More stuff in foo
    | * 72072a1 Initial commit foo
    * bfcb339 Initial commit in mono

[0]: https://github.com/josh-project/josh

[1]: https://manpages.debian.org/testing/git-man/git-subtree.1.en...

In target repo you create a folder and in that folder you rebase your dependency repository.

Maybe I can find better documentation that I remember writing it down somewhere.