Yiden have been "converting" nigunim from goyim for centuries (and they have been taking from us for forever). It's just now in the youtube era we find the sources and many seem unsavoury (like the band playing MBDs "Yidden, Yidden".
"Napolean's march" is lost to history but not in Lubavitch who since the Alter Rebbe sent a chossid to spy out to find out his marching tune, have been singing it regularly especially at the end of Neilah!
There was a Muslim imam in Dagestan named Shamyl who's song was taught by the Lubavitcher Rebbe zy"a and is sung today at farbrengens.
There is a shepard's LOVE SONG that the Kaliver Rebbe changed a few key words (i.e Rose (the name of the shepherd's object of love) to Sh'china, and the big forest to golus etc.) and now we have a Yiddish song longing for geulah. (here:
http://www.zemirotdatabase.org/view_song.php?id=81 - note that many of the websites say that the rose in the original song refers to the flower, but that makes no sense because what kind of longing is there with a rose in a forest. The original was obviously a girls name).
The French national anthem was "commandeered" by the Lubavitcher Rebbe zt"l on Simchas Torah 1975 and has been sung as a Lubavitcher niggun ever since.
This list could go on for many pages but I will suffice it with what's been said.