Just open a Depot in Germany and you can trade in Franffurt, Xetra, Munich, Berlin or wherever the Stock is listed. Transfer Money to the Account and start trading. Same thing we do, by opening an US Depot (ETrade or else). That gives us the advantage of trading for almost 18hrs a day. ;-)
If you use an online provider, you have to trade using a Limit Buy and for Term select Extended Hours. Afterward, you'll need to read a disclosure and press OK that you understand. From what the broker told me though, there are several risks involved. You are trading with more experienced traders and also it doesn't have the volume that regular hour traders have so the price flucuates more rapidly. Be careful if you decide to do this.