Adding formula to excisting formulas

[p45cal]

Try this on a copy of your workbook. It works on the Active workbook, so beware!:

Sub addIfError()
For Each sht In Worksheets
  For Each cll In sht.Cells(1).SpecialCells(xlCellTypeFormulas).Cells
    cll.Formula2 = "=Iferror(" & Mid(cll.Formula2, 2) & ","""")"
  Next cll
Next sht
End Sub