1-6 you can already do by setting the range when you create the assignment. If you need 1.1, 1.2 etc then you could go for 1-60 I guess. The letter grading isn't there. You might want to create a uservoice for that idea. I am sure quite a few people would like to see this.
@Marius Johansen Yes I agree. The assignment tool in Teams should have customizable grading scales. F. ex. A-F, 1-6, or anything the teacher wants to use as an expression for the grading of the assignment. Flexible and customizable grading options are IMO crucial for Teams to be considered as alternative to LMS.