In the movie "Das Boot" one of the crew member was a german expat, who went to Germany to fight for "his country" in WW2. Was this a common occurence ?

There have been several previous threads here on the subject, though mostly prompted by a Band of Brothers episode. This is not to discourage further questions and discussion. (I believe that I found these using the Reddit search box for "american german soldiers". Reddit searches can be useful.)

"In the series Band of Brothers during the post D-Day operations one of the German prisoners turns out to have been an American who had gone to Germany at the beginning of the war. Was this common?" at https://www.reddit.com/r/AskHistorians/comments/1f43i8/in_the_series_band_of_brothers_during_the_post/ , mentioning the case of Haupt in "ex parte Quirin".
"How many US citizens of German descent fought for Germany in WW2? Thinking of this scene in Band of Brothers where the American soldiers meet a German POW... who speaks perfect English." at https://www.reddit.com/r/AskHistorians/comments/21l5v8/how_many_us_citizens_of_german_descent_fought_for/ : /u/coinsinmyrocket 's reply, with several names but "no hard number exists in regards to the number of American citizens (either by birth or naturalization) who fought for Nazi Germany"
"In the second episode of Band of Brothers, one of the characters encounters an American-born German soldier. Are there any actual records of Americans with German ancestry returning to Germany to fight for the Axis powers?" at https://www.reddit.com/r/AskHistorians/comments/23uq1t/in_the_second_episode_of_band_of_brothers_one_of/ : /u/Superplaner cited a source saying that 5 American Germans were in the Waffen SS.
"In Band of Brothers, there is a German soldier born and raised in Oregon, but is serving in the Nazi military because his family "answered the Aryan call," or something along those lines. Was there an actual call to German, "Aryan" descendants to return home?" at https://www.reddit.com/r/AskHistorians/comments/54c0td/in_band_of_brothers_there_is_a_german_soldier/ : /u/JohnMLTX, TitusBluth, and Georgy_K_Zhukov. It also points to several threads that I cited above.