Currently the only way to register new workspace is to define it in module descriptor and let ModuleManagerImpl handle the registration at startup of Magnolia in its private methods.
It would seem more appropriate to move this code to RepositoryManager or to Provider as need to register new workspace is not specific to ModuleManagerImpl.
This change is specially relevant since deprecation of Data module as it prevents creation and registration of new workspace and content apps connected to them without restart of Magnolia.