Is there a way to add a task on a specific work day?
For example: I need to have a task that is done every next to last day of the month, in other words Workday -2.
That's unfortunately not possible. You can set the task to "every last day" which will recur on every last day of the month, or "every 28" or "every 4th Friday", but it can't be set to every 2nd to last day of the month that falls on a workday. We may consider adding such option in the future, though.