How do I insert a letter in sequence after my 'added' 001?

[RoastedApplePie]

I have multiple files I need to rename as a number and letter, for example 001a-z, 002a-z, etc. The issue is two fold (or maybe 3 lol). I need to mark specific files as 001a-z but I also need them to be lettered in a specific order, but this order is different every batch of files I need to rename.. I also have numbers in some of these file names, so I'm very CONFUSED! on how to efficiently rename and organize these files without doing them manually 1 by 1.

I've figured out you can add the numbers to the prefix of the files which solves one problem, but the second problem still remains, I need to letter SPECIFIC documents as 001a, 001b, 001c, and soforth.

So example a batch of files I need to rename look like this:
DOE, JOHN - SAMPLE 2000 DATA.pdf
DOE, JOHN - OTHEROTHEROTHER SAMPLE DATA.pdf
DOE, JOHN - OTHER SAMPLE DATA.pdf
DOE, JOHN - OTHEROTHEROTHEROTHER SAMPLE DATA.pdf
DOE, JOHN - OTHEROTHER SAMPLE DATA.pdf

and I need them like this.

001a. DOE, JOHN - SAMPLE 2000 DATA.pdf
001b. DOE, JOHN - OTHER SAMPLE DATA.pdf
001c. DOE, JOHN - OTHER OTHER SAMPLE DATA.pdf
001d. DOE, JOHN - OTHER OTHER OTHER SAMPLE DATA.pdf
001e. DOE, JOHN - OTHER OTHER OTHER OTHER SAMPLE DATA.pdf

The next batch of files will be 002a-z. 003a-z and so forth.

I feel like this is too much to ask, but is there a direction someone can lead me in to figure this out? THANK YOU SO MUCH IF YOU CAN HELP!!!

[Admin]

Hi, Numbering -> add a counter alphanumberic at pos