How do I create a script to create/modify local admin accounts?
This is what I came up with:

It also uses http://magervalp.github.io/CreateUserPkg/

#!/bin/bash
# This script will first check existing accounts for presence of USERNAME or admin    or administrator
# If found, it will change the password to PASSWORD
# If none are found, it will run the package create_USERNAME-1.0.pkg which creates an account with Full Name = Administrator, Account Name
# (short name) USERNAME with the password.

function checkusername () {

local test1=$(dscl . -list /Users | grep -i USERNAME)
local test2=$(dscl . -list /Users | grep -i admin)
local test3=$(dscl . -list /Users | grep -i Administrator)

if [ "$test1" = “username” ]; then
  echo the username username already exists
  echo setting password
  dscl . passwd /Users/username PASSWORD
  exit 
else 
  echo
fi

if [ "$test1" = "Username" ]; then
  echo the username Username already exists
  echo setting password
  dscl . passwd /Users/Username PASSWORD
  exit 
else
  echo 
fi

if [ "$test1" = “UserName” ]; then
  echo the username UserName already exists
  echo setting password
  dscl . passwd /Users/UserName PASSWORD
  exit
else 
  echo
fi

if [ "$test2" = "admin" ]; then
  echo the username admin already exists
  echo setting password
  dscl . passwd /Users/admin PASSWORD
  exit 
else 
  echo
fi

if [ "$test2" = "Admin" ]; then
  echo the username Admin already exists
  echo setting password
  dscl . passwd /Users/Admin PASSWORD
  exit 
else 
  echo
fi

if [ "$test3" = "Administrator" ]; then
  echo the username Administrator already exists
  echo setting password
  dscl . passwd /Users/Administrator PASSWORD
  exit 
else 
  echo
fi

if [ "$test3" = "administrator" ]; then
  echo the username administrator already exists
  echo setting password
  dscl . passwd /Users/administrator PASSWORD
  exit 
else 
  echo
fi


}

checkusername

/usr/sbin/installer -pkg 'create_USERNAME-1.0.pkg' -target /