Hey IHD,
first off good luck with the training ahead of you. It has been about four years since I seriously looked into converting. For some reason in Germany many employers don't like seeing an LBA license based on an FAA license. What I was going to do and seemed to be the quickest (not necessarilty the cheapest) was convert my FAA PPL/IFR to an LBA PPL/IFR. This is a very simple conversion. You basically just have to find an IP who will sign you off after doing a three leg cross country flight. Herr Wakawitz at Ikon is very familiar with this conversion.
Once you have your PPL/IFR, sign up for the CPL course. Do it from scratch without being based on your FAA. That way you learn all the German/European regs, etc. The course takes 4-6 weeks. Once you have that you are almost done as the ATPL 1 in Germany is only a written exam. (Only does not mean easy ie very hard)
I got most of my info from Fr. Feiertag at the LBA in and Dr. Kotzohr at Ikon. I have heard of Comfly and they are probably great too. I do know that Ikon has a close relationship with the LBA examiners and almost everyone passes without a problem. Here is their web site:
www.ikon-flugschule.de