Visual Basic

foreach tabpage in tabcontrol
    foreach control in tabpage
        if control is combobox  (skipping the other controls, e.g. buttons)
            do what needs to be done

Sub fill(ByVal tbcon As TabControl)

       Dim cnt As Control
       For Each cnt In tbcon.Controls

           If TypeOf cnt Is ComboBox Then
               Select Case Tag
                   Case 0
                       cnt.Items.Add("NP")
                       cnt.Items.Add("ND")
                       cnt.Items.Add("FP")
                       cnt.Items.Add("FD")
               End Select
           End If


       Next
   End Sub

Sub fill(ByVal tbcon As TabControl)
    Dim cnt As Control
    For Each cnt In tbcon.Controls
        If TypeOf cnt Is ComboBox Then
            Select Case CType(cnt, ComboBox).Tag
                Case 0
                    cnt.Items.Add("NP")
                    cnt.Items.Add("ND")
                    cnt.Items.Add("FP")
                    cnt.Items.Add("FD")
            End Select
        End If
    Next
End Sub

dim cmbcntrl as combobox
 Dim combos() As String = New String() {"NP", "ND", "FP", "FD"}
dim i as string
For i = 0 To 11
            cmbcntrl = cmbcntrl.Items.AddRange(combos)
next

 Dim combos() As String = New String() {"NP", "ND", "FP", "FD"}
   Dim i As Integer
dim cmbcntrl as combobox
 For i = 0 To 11
            cmbcntrl = cmbcntrl.Items.AddRange(combos)

Dim arr() As String = {"NP","ND","FP","FD",...}

For Each tabPage In TabControl1
  For Each control In TabPage
    If control Is ComboBox Then
      control.Items.AddRange(arr)
    End If
  Next
Next