Here is the code, all I need to do is add another column to it (columns C and
D). Any tips is greatly appreciated... been searching the net for answers
but can't figure it out.

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngDV As Range
Dim oldVal As String
Dim newVal As String
If Target.Count 1 Then GoTo exitHandler

On Error Resume Next
Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
On Error GoTo exitHandler

If rngDV Is Nothing Then GoTo exitHandler

If Intersect(Target, rngDV) Is Nothing Then
'do nothing
Else
Application.EnableEvents = False
newVal = Target.Value
Application.Undo
oldVal = Target.Value
Target.Value = newVal
If Target.Column = 3 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ", " & newVal
End If
End If
End If
End If

exitHandler:
Application.EnableEvents = True
End Sub