Yes, you can but there are a few steps in order to complete this.

1. Export all the assignments with grades via the Gradebook Print out found via
  • Gradebook
  • Reports
  • Gradebook Print out

2. Create a Letter Grade Scale as per Letter Grade Scale

3. Remove all assignments from the Gradebook as per How can I change my grade book setup? 
4. Add the Letter Grade Scale to the Assignment Types in the Gradebook
5. Re add the Assignments as per How can I change my grade book setup? and re input the Grades as Letter Grades