Not really. The Alps would be a barrier from them conquering Germania from that direction. Then logistics and communication would be a nightmare...and assuming they did, Gaul would be completely surrounded....yeah no.
EDIT: They can go from the Danube, but even that doesn't make much sense or have any real plausibility. It's just a wide open plain, with few defensible areas from the eastern side, and theres not much the Romans want. So no dice.