The numerous instances where originally Allied personnel fought in Axis units is quite famous-most notably large amounts of Russians, Ukrainians, Cossacks, and other Soviets but also continengents (mostly in the Waffen-SS) from France, the Netherlands, Norway, and even Britain and India.
But even a google search didn’t turn up anything for the reverse-were there any Axis troops who got used by the Allies in World War II?
There was a documentary about it. Site claims “[a]s many as 10,000 Germans and Austrians fled Nazi persecution and joined British forces to fight against Hitler.”
There were plenty of German-born troops (or children of Germans) in the US Army. I don’t know if they were forced to fight in the Pacific (as those Japanese descent were restricted to European theatre)
I am currently reading two WWII books Anthony Beevor’s “History” and William Manchester’s “Churchill-Defender of the Realm”. But one of them, probably Beevor, mentions Italian units being used
in the Italian campaign. Of course Italy had overthrown Mussolini and switched sides by then.
There was Hitler’s nephew William Patrick Hitler, who served in the U.S. Navy as a pharmacist mate.
But then he was born in Liverpool, Great Britain and didn’t move to Germany until he was 18.
That wasn’t uncommon.
Romania also flipped sides and sent over 500,000 men to swell the ranks of the Soviet army in the east.
Bulgaria never declared war on the Soviets (despite being in the Axis camp). It also never sent any troops to the front. In September '44 the Soviets declared war on Bulgaria and within three days a peace treaty had been worked out. On the fourth day Bulgaria was allied with the Soviets and sent nearly 500,000 men into Yugoslavia to fight the Germans.
That Wiki article says he named his first son “Alexander Adolph”. I wonder if anyone said “ixnay on the Adolphay” 
You can find some good books on the Spanish Blue Division which fought valiantly in Russia with a lot of success. Technically Spain was a neutral country but with very strong ties to Germany.
Try this website for links and books.
And you are right, finding info in these guys is pretty hard.
Never knew about that Hitler nephew. Cool.
Not a military unit, but the Soviets set up a National Committee for a Free Germany and a League of German Officers as political units.