To get a large migration it would really have to depend on the extent of a German victory. The Germans signed repatriation agreements with the Soviet Union and Italy that allowed ethnic Germans to be repatriated from the Baltic States, Bessarabia and South Tyrol to the German Reich. In the unlikely scenario of a complete German victory over the United States, I would assume the Germans could forcibly repatriate ethnic Germans from the United States (and Canada too). Though such a scenario is ASB territory.
The other question is who exactly is German enough to settle in Germany? According to the 1940 census, there were 1,589,040 people in the United States using German as their primary language. This was a decline from 1930 where there were 2,188,006 German speakers registered. In 1930 there were 1,608,814 persons born in Germany and 370,914 born in Austria. Of these the 2/3 arrived before 1910.
Let us assume that the United States remains neutral, since IMHO that would be more plausible. Germany could attract some German immigrants and their children to return, especially if the U.S. economy remains mired with high unemployment. It was not until 1942 that U.S. unemployment decreased by a great deal, Germany by contrast had a much steeper and earlier decline in unemployment (mostly due to massive government spending on rearmament and public works). One would assume with all their new conquered lands the Germans unemployment would remain low for the foreseeable future.
German Unemployment
1933 26.3%
1934 14.9%
1935 11.6%
1936 8.3%
1937 4.6%
1938 2.1%
1939 0.5%
U.S. Unemployment Rate
1933 24.9%
1934 21.7%
1935 20.1%
1936 16.9%
1937 14.3%
1938 19.0%
1939 17.2%
1940 14.6%
1941 9.9%
1942 4.7%
Now on plausible return migration. I could not find figures for Germany for the depression. I was however able to find those of Sweden, a country with a similar emigration pattern to Germany's. Between 1930 and 1939 30,639 Swedes left the U.S. for Sweden. By contrast, a mere 3,960 Swedes emigrated to the U.S. between 1931-1940. This is significant because Swedish return migration had historically been low, with only around 15% of Swedes returning (the average for all Europeans being around 35%). However, Sweden fared relatively well throughout the Great Depression. This made enough Swedes feel they'd be better off back home. Presumably some Germans could be persuaded of the same.
If we consider that German and Austrian emigration to the United States had peaked in the 1880s and been declined to a trickle after WW1, I would estimate you could get a maximum of 100,000 Germans and German-Americans to return. This would probably require economic incentives such as free passage, guaranteed work and possibly tax holidays. However, I doubt many would want to go to the conquered territories. As with most return migrants, they would want to return to their areas of origin in Germany.