How to `git submodule add` Existing sub Repository?

Question

The Question

How to add existing sub repository as a submodule in git?

The Why

I have a private codespace supermodule with submodule

Answer 1

1. remove files from index foo/bar: git rm -f --cached foo/bar
2. git submodule add https://github.com/octocat/Spoon-Knife.git ./foo/bar