I'd be interested in knowing this too! I haven't found a way, but if there is, I'm all ears!

I agree that this should be available. Other programs import grades directly into Google Classroom. 

One of the reasons that this is important aside from the obvious ease of adding the assessment is that students see the result in their classroom result and remember that it is there. I am finding that for the students that need this the most, it tends to be out of side, out of mind. As a result, I spend so much time chasing them to do the assigned tasks and then I have to go back again to find out if indeed they completed the task.