This cannot be done with a field, automatic numbering is sequential by design; vba would be required.
In Excel I accomplished this by a cell at the bottom of the column that has the ID numbers which says MAX of the column +1. I then use a macro to insert a new row above the current one and put the value found in the bookmark for the formula in the ID column for the new row.
Here is the vba code to assign the new number to column Q of the new row:
Code:
' Insert new row above selection
'
Dim lRow As Long
lRow = Selection.Row
Selection.EntireRow.Insert
'
' Assign Case Number
'
Dim lNext As Long
lNext = Range("Next_Number").Value
Cells(lRow, "Q").Value = lNext
The cell with the MAX formula has the label "Next_Number."
I prefer using Excel for data rather than Word and wouldn't know how to get the MAX function to work in Word. I'm sure that something could be done using vba.