The source for the middle name of Hanssen is only family records. I am helping my aunt with all of this who is the daughter of Virgil Foss (one of Han's sons). I have gotten most of my other information from the daughter of Han's other son Harlan. Much of it is written correspondance from 30 years ago.
I appreciate your time and help. I will keep looking for other sources of information and see if I can get a copy of the naturalization information from South Dakota.