But this limitation is only db-related, right? If a package is opened from a file and added to a project, it can later be opened via project items to be perfectly displayed as spec and body.

This should be improved, because this limitation to "db-project-items" is not consistent and not obvious, too.

And how is it possible after opening e.g. via "View Spec & Body" from db to decide, whether the project item should be either spec or body?

If I do Project > Add to Project, I am not able to identify what exactly I put into the project. Switching between the spec and body tabs in the main package view doesn't change anything and for both the menu says after adding Project > Remove from Project. The project browser additionally states an unclear "Package ...".