Ne(i)ls Kopperud and Mary Hanson

I am trying to find more about my great-grandparents. Here is all I know and I can't get back any further than this: Nels Kopperud born 24 November 1853 in Lier, Hordaland, Norway and died 10 June 1918 in Watertown, South Dakota. The Kopperud name was the name of the farm to which the family moved in Norway. Otherwise the last name would be Nielson. Mary Hanson born 28 January 1857 in Drammen, Buskerud, Norway and died 3 April 1920 in Rochester, Minnesota. I know they both immigrated through Milwaukee and lived there several years as Nels worked for Mary's brother, Captain Ole Hanson, and he worked on the Great Lakes. In 1879 they moved to a homestead near DeSmet, South Dakota. Any and all help would be much appreciated! Thanks.