Now thats a different approach to the subject, and conversely for inserting extra columns it could be

Sub Tbl_Add_ColumnsII()
    Dim oTbl As Table
    Dim oCol As Column
    Dim lngSeq As Long
    Application.ScreenUpdating = False
    Set oTbl = Selection.Tables(1)
    lngSeq = 2 'Use 2 to insert row after each odd or even Column. Use 3 for every third, 4 for every fourth, etc.)
    Set oCol = oTbl.Columns(lngSeq) 'For every odd Column
    'Set oCol = oTbl.Columns(lngSeq + 1) 'For every even Column, or other sequence (e.g., every third, fourth, fifth etc.)
    Do
         oTbl.Columns.Add oCol
         On Error GoTo Err_Index
         Set oCol = oTbl.Columns(oCol.Index + lngSeq)
    Loop Until oCol.Index = oTbl.Columns.Count
    oTbl.Columns.Add oCol
    lbl_Exit:
    Exit Sub
    Err_Index:
    If oTbl.Columns.Last.Index - oCol.Index = lngSeq - 1 Then
         oTbl.Columns.Add
    End If
    Resume lbl_Exit 
End Sub