C# と VB.NET の質問掲示板

Owner Drawを使うとよいかと思います。

サンプル:
Public Class Form1
    Private _List As List(Of clsItem)

    Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles Me.Shown
        _List = New List(Of clsItem) From {
                    New clsItem() With {.ID = "a", .Name = "aaa"},
                    New clsItem() With {.ID = "b", .Name = "bbb"},
                    New clsItem() With {.ID = "c", .Name = "ccc"}
                }
        ComboBox1.DataSource = _List
        ComboBox1.DisplayMember = "Name"
        ComboBox1.ValueMember = "ID"
        ComboBox1.DrawMode = DrawMode.OwnerDrawFixed
    End Sub

    Private Sub ComboBox1_DrawItem(sender As Object, e As DrawItemEventArgs) Handles ComboBox1.DrawItem
        Dim itm = TryCast(ComboBox1.Items(e.Index), clsItem)
        If itm Is Nothing Then
            Exit Sub
        End If
        Dim TextDraw = String.Format("{0} - {1}", itm.ID, itm.Name)


        e.DrawBackground()
        e.Graphics.DrawString(TextDraw, ComboBox1.Font, Brushes.Black, e.Bounds)
        e.DrawFocusRectangle()
    End Sub


End Class

Public Class clsItem
    Public Property ID As String
    Public Property Name As String

    Public Overrides Function ToString() As String
        Return Name
    End Function
End Class