Really bad way of adding a letter after the first character in a username. Makes username validation a little easier if your client is using the format {f}{m}{last}@acme.com.

usergen.py

# Usage: python3 usergen.py -f users.txt | tee modified-users.txt

import string
import argparse

parser = argparse.ArgumentParser()

parser.add_argument("--file", "-f", type=str, required=True)
args = parser.parse_args()

file = open(args.file, "r")

for user in file:
    for letter in string.ascii_lowercase:
        modified = user[:1] + letter + user[1:]
        username = modified.rstrip()
        print(username)