Hello,
You would do that from the templates.
/usr/local/directadmin/data/templates/virtual_host*.conf
remember to first copy them to the custom directory and edit the new ones in there. Else they'll overwritten when DA updates.
then just type:
echo "action=rewrite&value=httpd" >> /usr/local/directadmin/data/task.queue
to rewrite the user httpd.conf files with the new custom template.
John