Does anybody know this answer?
I'm not sure this is what you are looking for... but in the Gradebook you can add an item and leave comments for the student to read. Example: you have a skill that you put in as gradebook item, say injections, and in the Grader report you can enable quick feedback which puts a text box below each "grade" that you can fill in with comments. Pretty slick.
I have Moodle 1.9 I don't know the differences but Grades = Gradebook. Click on Grades in the Administration block. From here I click on the upper-left corner to the "Choose an Action". Here you can select "Edit: Categories and Items" to create you "Items" for putting in the feedback.
1.8 most likely is different so I really can't answer to that but as long as you have an item (or offline item) in the gradebook, you can add feedback which is what you are really after.
One last thought... check YouTube. Put in "Moodle Gradebook 1.8" and you actually will get some video assistance.