Can I supplement my unofficial transcripts with missing context?
I don't think there is any rule that would suggest it's bad practice to add the scorecard to your unofficial transcript. However, I would say that it is unlikely to change much. If you think it is important to address the grades, I'd rather find a way to weave this into the letter or any other free text that will accompany your application. This is likely a much better way to put your grades into context. Consider that even the percentages tells you next to nothing. In the US system see here a D generally ranges between 60-70%, so in comparison to your table it should be considered better than average? But nevertheless, if you apply will all Ds or even all Cs in the US system you will likely be dismissed out of hand. So in my eyes, a personalized note would be needed to convey the nuances about your grading system.
Of course, I am not sure if your applications offer up space for such a section.