If you go under settings you’ll find the option to import projects that already exist on your PC. For that you’ll first have to choose a scanner.
Before you are able to import projects you have to select your scanner.
You can find this option in Settings -> External Programs -> Scanner.
After you chose a folder and a database, your projects will be imported.
This can take hours depending on how many projects you have to migrate. On any stage, this can be interrupted by closing the Manager and resumed by repeating the steps from above.