What you could do, without the need for a separate column in your table, is use the mergefield \f switch to output the ')' for each populated line. That wouldn't help with lines that wrap, though, and there's no way for a mailmerge to generate the ')' in such cases. The alternative would be to use a macro to populate the column with ')' characters post-merge. The following macro will both execute the merge and update the output document post-merge.
Code:
Sub MailMergeToDoc()
Dim Sctn As Section, sngHght As Single
ActiveDocument.MailMerge.Execute
For Each Sctn In ActiveDocument.Sections
With Sctn.Range.Tables(1)
sngHght = .Cell(1, 1).Range.Characters.Last.Previous.Information(wdVerticalPositionRelativeToPage)
With .Cell(1, 2).Range
.Text = "))))"
Do While .Characters.Last.Previous.Information(wdVerticalPositionRelativeToPage) < sngHght
.Characters.Last.InsertBefore ")"
Loop
End With
End With
Next
End Sub
For PC macro installation & usage instructions, see:
http://www.gmayor.com/installing_macro.htm