No, you cannot associate a selectbox grade item with a Submission folder, or with any other D2L tool. The only type of grade item that you can associate with a Submission folder, or any other tool in D2L (i.e., Discussions or Quizzes), is a numeric grade item. Numeric grade items allow you to grade users by assigning a value out of a specified total number of points (e.g., 7/10).
For additional information about grade items and associating grade items with D2L tools, please see our Grades guide, Submissions guide, Discussions guide, and Quizzes guide.