No, the autonumber won't store letters in its field. Add another field to
your table that will hold the user's initials, and then write those initials
into the record when you create the record.

If you want to display the combination of the autonumber value (hint:
autonumber values are best used "in secret" and not shown to users) and the
user's intials, you can always concatenate the two values into a single
string value:

   [autonumberfield] & [userinitials]