Hi Laci:
Short answer, yes, you can. Longer answer: Google Voice is a US-only service, which forwards to US-based phone numbers. If you want to continue to manage your account, listen to voicemail messages from a forwarding phone, or use mobile text messaging, you'd need to have a US-based forwarding phone listed on the account.
If you don't periodically place or receive calls, or send or receive text messages using your Google Voice number, Google will reclaim it (take it back) as unused, so that it can be reissued to another active user.
You can certainly add (or switch to) a SIP VOIP service provider that offers service in Germany. If you have an OBi 200 or 202, it can be configured with up to four different service proviers; OBi 100 or 110 products can use up to two providers. You can shop among the service providers listed on Obihai's "OBiTALK Compatible Service Providers" page, or do a web search for providers.