If you are all in Germany it seems like the easiest option would be to file an adoption case there.
Any US State is very likely to decline to exercise jurisdiction unless the child is physically present in that State. Virtually every US State requires a "home study" prior to granting an adoption petition, and it is not practical for a US Court investigator to conduct a home study in Germany.
Arizona does not appear to have jurisdiction unless you are residing there.
An Arizona statute
A petitioner may begin adoption proceedings in the court in the county where the prospective adoptive parent
resides or, if applicable, in the county where the child is a ward.
My opinion is that you need a lawyer in Germany to do this unless you want to wait until you are back in the States.
You can find adoption lawyers in Germany listed by location athttp://www.hg.org/law-firms/adoption/germany.html
I understand that this may not be the answer you had hoped for, but it would be a disservice to you, and unprofessional of me, not to provide accurate information.
I hope this information is helpful.