New grades are introduced all the time e.g., https://en.wikipedia.org/wiki/General_Certificate_of_Seconda...
> In 1994, the A* grade was added above the grade A, to further differentiate attainment at the very highest end of the qualification. This remained the highest grade available until 2017.
The grades are now 1-9, with 9 being the highest and above A*.
In this particular case, I think it certainly is within a University's remit to award a special grade given the exceptional circumstances.