If you currently have a large /usr partition, and /home is mounted onto a smaller / partition, you can move /home into /usr/home.
mkdir -p /usr/home cp -Rp /home/* /usr/home/ mv /home /home.old ln -s /usr/home /home
Test out everything, and make sure it all still works correctly.
Once satisfied that it's all working, and positive that the new location is being used, then you can delete /home.old:
rm -rf /home.old
Don't delete the directory yet, if you have any doubts.
You'll need to ensure that /usr has quotas enabled on it, and that the:
quota_partition=/usr
is set in your directadmin.conf.