Can I add my project to an external Git repository and keep it within Platform.sh?

Yes, as Platform.sh is a Git-based infrastructure, it's entirely possible to clone your project over to a repository such as GitLab, GitHub or Bitbucket and also keep your Platform.sh Git repository.

But that isn't the recommended approach. The simplest approach is to use one repository for your code, by either using your Platform.sh repository or through external source integrations.

Note that if you choose to keep both an external repository and your Platform.sh repository, you have to manually update your project and/or repository when making changes.

Git includes the "remote" command to manage multiple remote repositories. Find out more in the Git documentation.