You can either buy a ticket from/to the border to cover the route in Germany. What you need depends on the train you plan to use.

You can also exchange your pass for a global pass (4 travel days minimum). If you bought it from Interrail, see the exchange process:

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

OR-depending on to where and how much you also want DE=Germany, buy an extra pass for that country. But most likely-as both are quite big and rail-expensive countries, may be less cost-effective as doing the exchange to global and coughing up the fee for that