I want /home to be mounted on /usr

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.