adduser username
qmail-pw2u </etc/passwd >/var/qmail/users/assign
qmail-newu

/var/qmail/control/virtualdomains - add domainname.ext:user (for the catchall account there)
/var/qmail/control/rcpthosts - add domainname.ext
/var/qmail/control/locals - add domainname.ext

~username/.qmail-info for the info account in that userid
~username/.qmail-default for the catchall account