+= のこと

無理矢理ジェネリック拡張メソッドを書いてみた
<Extension> Sub [AddHandler](Of T As Class)(ByRef x As T, y As T)
    Dim xx As [Delegate] = DirectCast(DirectCast(x, Object), [Delegate])
    Dim yy As [Delegate] = DirectCast(DirectCast(y, Object), [Delegate])
    x = DirectCast(DirectCast([Delegate].Combine(xx, yy), Object), T)
End Sub

' 使い方
Dim fn As Action = AddressOf Do1
fn.AddHandler(AddressOf Do2)
fn()