action script 2.0 extending Button

[thosecars82]

Hello there
I have this class:

class TestButton
extends Button
{
public function onRollOver()
{
trace("onRollOver");
}
}

I want to create an instance of this class by using the flash 8 environment. Nevertheless, when I choose convert to symbol, I just can choose among Graphic, Button and MovieClip. Does anyone know how could I create a shape liek a square and make it an instance of TestButton?
Thanks in advance