You might want to script it yourself or hire somebody, see an example by a link http://help.directadmin.com/item.php?id=273
Some kind of a lock file, will deny running multiple backup tasks.
The all_pre.sh is only for DA commands in the interface, and doesn't account for cron backup creation (probably best to use both)
Or... another solution which won't ignore other backups, is to just ensure only 1 instances of the dataskq is running at a time. http://help.directadmin.com/item.php?id=246
This will keep other backup requests in the task.queue file until the next instance of the dataskq runs.. so no backups are skipped/missed.
Well, its ok for me atm, but maybe something more dedicated just on backups should be better cause backups require just more resources then a restart of a service