Changing the username, UID and GID default rules on linux

I always forget this one but certain companies I work for have a username, UID standard which doesn't comply with the POSIX norm.
I've seen several people on forums saying they have hacked the tools to accomplish this but there is no need. Just edit /etc/login.defs

BTW, to see what files useradd looks at, just use :-

strace -e trace=file useradd somebody