There's an excellent document from Rutgers about how to use the Assignments tool to provide students with grade feedback: https://sakai.rutgers.edu/helpdocs/assignments.html#6
Because the grading for an assignment is done within the Assignments tool it is not possible to override the grade from within the Gradebook.
In order to circumvent this, we recommend that you disable the "Add Assignment to Gradebook" option and simply add the assignment item as a new item in the Gradebook. This will allow the instructor or TA to run down the roster and add grade information for each student within the Gradebook tool.
Additionally, there are tools that you can use to download a zip package of all students' assignment information, which can be manipulated and re-zipped into the Assignments tool with all feedback and grade information in tow. More information is available here: http://support.ccnmtl.columbia.edu/knowledgebase/articles/9865