Yes, you can top it up at https://www.aufladen.de/en, although the minimum top-up is €15. If you're going to go that route, I recommend using their "direct" option for €0.50 more, they will input the activation code for you so you don't have to fumble through it.

Alternatively, put your SIM card in your Droid every couple of months while you're at home and send yourself a text message for €0.49. That will reset your 6 month clock.