I have posed your question to the service team of Deutsche Bahn on Twitter, and the answer is yes.
Q: Kann man von DB-Bahnhöfen aus TGV-Tickets kaufen? [...]
A: Ja, das geht. Am besten geht er in ein DB Reisezentrum. /jn
Translation (by me):
Q: Is it possible to buy TGV tickets at DB train stations? [...]
A: Yes, that's possible. It would be best if he went to a DB Reisezentrum (DB travel service center) [to buy a TGV ticket in Germany]. /jn
You can use this search engine to find a Reisezentrum near one of your way points on your Germany trip. Put in a City you'll be staying at and look at the Reisezentrum section in the results. Without looking, I'd bet that all the cities you mentioned are big enough that at least their main stations provide a Reisezentrum.
A Reisezentrum usually accepts most payment methods, like cash, european debit cards (Maestro/EC) and common credit cards.