The field TRACK is defined by the id3 standard to be numeric with the exception of the slash to separate the total number of tracks.
So, adding a character prefix would create non-standard contents.
I don't know whether such a format would cause hicups in players - but it is non-standard.
If you still want to get a character in front, you could try - after you have used the auto-numbering wizard - an action of the type "Format value" for TRACK
Format string: A%track%
to get the letter "A" in front of the number.