If there isn't a way to do this with TokenNameNumber, you should be able to pull it off with the MetaScriptToolbox.
You'd just use a character ability to store the translation from number to letter. I'll use the character PublicTables, and I'll call the ability IndexLetters. The contents look like this:
0=A
1=B
2=C
3=D
4=E
5=F
6=G
...etc.
Take that out as far as you need to -- that is, as many tokens as you need to name in one go.
Then, with the MetaScriptToolbox installed, you can select the tokens you want to name and run:
!forselected(^) token-mod --set name|"?{Root name?} (get^.PublicTables.IndexLetters.{&i}/get)"
That would prompt you to enter the root name (ie, "Vampire Spawn") and would translate the index of the token (specific to *THIS* running of the command... that is, every time you run this command you're starting over at 0) into a letter.
That produces tokens with names how you want: