I'm not sure what you mean. All you have to do is put a symlink in place of the normal sendmail binary that points to /var/qmail/bin/sendmail, e.g.,

mv /usr/bin/sendmail ~/sendmail.old
ln -s /var/qmail/bin/sendmail /usr/bin/sendmail

(or wherever you have sendmail installed; some distros have it in /usr/sbin).

All will then work nicely, without changing anything else relating to sendmail.