There is an easier step,
- git clone <your_repository_to_import> from above the monorepo
- under your monorepo, yarn lerna import <path_your_repository_to_import>
- git push
The project at https://github.com/pingleware/bestbooks is a prime example of converting a multirepo to a monorepo,
- git clone https://github.com/pingleware/bestbooks
import https://github.com/pingleware/bestbooks-core
- git clone https://github.com/pingleware/bestbooks-core
- cd bestbooks
- yarn lerna import ../bestbooks-core
- git push
- rm -rfd ../bestbooks-core