Private Sub Worksheet_Change(ByVal Target As Range)
' Developed by Contextures Inc.
' www.contextures.com
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
'      NOTE: you can use a line break,
'      instead of a comma
'      Target.Value = oldVal _
'        & Chr(10) & newVal
      End If
    End If
  End If
End If

exitHandler:
  Application.EnableEvents = True
End Sub

http://www.contextures.com/excel-data-validation-multiple.html

Selecting different values from a data validation drop down list will populate the same cell, i.e. it will create a concatenating list separated by commas.
Amend the "If Target.Column" values to set the columns this applies to.

Excel (VBA) drop down list multiple values in single cell

list, excel

Visual Basic