I prefer this syntax. Instead of looping through each cell, just read the whole selection into an array, act on the array, then dump the array contents back onto the worksheet. A maximum of two touches of the worksheet.
Sub AddQuote()
Dim i As Long, j As Long
Dim inputData As Variant
Dim outputData As Variant
' only act on a range
If TypeName(Selection) = "Range" Then
' assign selection value to a Variant array
inputData = Selection.Value
' create an output array of the same size
ReDim outputData(LBound(inputData) To UBound(inputData), _
LBound(inputData, 2) To UBound(inputData, 2))
' loop through all array dimensions
For i = LBound(inputData) To UBound(inputData)
For j = LBound(inputData, 2) To UBound(inputData, 2)
' array element will be = Empty if cell was empty
If Not IsEmpty(inputData(i, j)) Then
outputData(i, j) = Chr(39) & Chr(39) & inputData(i, j) & Chr(39)
End If
Next j
Next i
Selection.Value = outputData
End If
End Sub
It also accounts for a multiple-column selection.