Modules & VBA :: Adding Time With Work Hours Function

In my DB that we use and a workflow tool, some of our work has and due date and time.If we get the complete_package our work time starts and we have X amount of time to complete our work. This is something i worked on but set it aside, now i am coming back to try and fine tune this so it returns a more accurate value.

So if we receive and [Date_Complete_Package_Received] at 09/13/2014 09:00:00 AM and based on the work being done we have 5 hours to complete the work, then the [Date_and_Time_Complete_Package_DUE] would be 09/13/2014 02:00:00 AM. That part is simple and i have coding that does that just fine. [SLA_Time] is where it gets the amount of time allotted, we have 5,10,12,14,and 16 hours depending on what is being done.

Code:
Me.Date_and_Time_Complete_Package_DUE = DateAdd("h", [SLA_Time], [Date_Complete_Package_Received])

This is done on AfterUpdate of a field on one of my forms and it works the way it is but what i need is to be able to run this through my Workhours Function so i am not getting values that our in off hours.The following doesn't work i know i can't use the Workhours function with the DateAdd but this is just to show what i am trying to do.

Code:
Me.Date_and_Time_Complete_Package_DUE = WorkHours(DateAdd("h", [SLA_Time], [Date_Complete_Package_Received]))

I have a Workhours module and it is used for a lot of things and it works perfectly.