Sorry, I don't know German yet. So, my communications will have to be in English.
It sounds like paypal handles German subscription payments differently than payments in the US.
Nice PayPal relies on PayPal's IPN messages to trigger the expiration process. PayPal sends an expiration IPN message to the Nice Memberships extension. Then Nice Memberships expires the users memberships. The expiration IPN message is part of the PayPal subscription payment system. Other types of PayPal payments do not provide expiration IPN messages. This means that to use another payment type, the Nice Memberships would need some heavy modifications.
I do have a version of the Nice Memberships that excepts off-line payments (checks, cash, etc.). The admin has to manually enter the payments. However, the version is a prototype and not ready for distribution. It does not have the proper expiration functionality to handle off-line payments. I don't think the new version will be ready for 6 months, maybe longer.
~ Best regards