However, there is a new feature in LimeSurvey 1.92 (which is fast approaching alpha/beta-testing phase) which does would let you do that.
Say you have 50 questions, each worth one point (and say the correct answer for each question is 1). You could create the following two questions, both of type "Equation":
score = sum(q1==1, q2==1, q3==1, ..., q50==1)
grade = if((score >= 1 and score <= 10), "A", if((score >= 11 and score <= 20), "B", ... , "F")))
For more details, you can read about
Expression Manager