There is no fixed list of accepted names in Germany like there is in Denmark, so it's up to the registrar to accept a name or to refuse it. Of course he has got books to look up the name, but even if he doesn't find it he can decide to accept it.
One point is the gender: at least one of the given names has to have a clearly recognizable gender. So a boy can have Maria as a given name (which used to be fairly common until maybe fifty years ago, think of the composer CarlMaria von Weber) or Eike (which can be both m. or f.), but he must have a second given name.
A second point is the well-being of the child, who should not suffer from his name. And this is where courts come in: if you don't accept the decision of the registrar you can bring an action against the state and sometime you will be successfull. There was a couple a few years ago who wanted to name their child Pumuckel, which is the name of a comic character and it sounds really rediculous in Germany. But they succeeded. Poor Pumuckel! So much about Cardboard.
But you can make up a name, if there are no bad associations going with it. We called our daughter Wiona, which is a mix of Winona and Fiona, and it was accepted.
The custom to name a child after the day's saint is now dying out in Germany except in some very conservative areas like in parts of Bavaria.