You can dynamically add comboboxes at runtime by declaring a new combobox, setting it's parent property to the container (i.e - panel, groupbox etc), then setting it's position and setting visible to true.
In VB.Net
Private intTop As Integer = 5
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim cboBox As New ComboBox
cboBox.Parent = Me
cboBox.Top = intTop
intTop += 5 + cboBox.Height
cboBox.Visible = True
End Sub
Hope this helps