Yes, you can apply if you have certifications for at least 2/3 of the credit points necessary for your Bachelor’s degree. In that case the current transcript of courses can be used for the application.
Note that all modules necessary to complete the Bachelor’s degree have to be finished before the start of the Master course, and certificates have to be submitted before the end of the first semester of the Master course.
If the docent of the module has no objections and there are no capacity restrictions for that module, as for seminars or practical courses, you may do so. After successful examination, please ask for a paper certificate, which you later can use for your Master studies. Note, however, that no legal claim for admission to the Master course can be founded on that.
It is recommended to re-register. In the case of a successful application to the Master course, the inscription can just be changed from "Bachelor" to "Master". In the case your application is rejected, you can still uninscribe in order to get full or partial refund of the paid fees, depending on the date of exmatriculation.