Hello Zoirreavaph86
You can purchase an Eurail Global Pass that covers Germany and Switzerland. I, personally, would not purchase it, however. The Eurail Pass is not very useful in Switzerland, as it does not cover all the routes a local pass does.
Simply book a (Supersaver) train ticket from Munich to Luzern. Then purchase the Bernese Oberland Pass, if you will spend the most time in the Bernese Oberland.
From your question, it is not obvious where your base will be. That’s an important decision to avoid too much travel between your base and the places of interest. Skip Interlaken, it is not a place one has to visit. It is a travel hub to reach the Jungfrau region (Grindelwald, Wengen, Mürren, Lauterbrunnen), or Lake Brienz.
Kind regards,
Roland