The historical drama was originally in development at FX but NBC picked up the show earlier this month.
The project, which is set in Missouri just after the Civil War, focuses on a soldier called Jason who travels across the country. He eventually settles in a town where he is considered to be the solution to the community's problems.
Deadline says that Wellin has now signed up to play a tough girl who lost her father and brothers in the war. Jason reportedly begins to look after her.
The Crossing has been penned by Northern Exposure scribe Josh Brand.