WHY? By adding another filter media to that filter you will increase the amount of junk getting caught and increase the chances of high nitrates. Bio balls are meant to work aerobically as water falls over them, similar to the bio-wheel. submerged, I do not think that would do anything. even adding live rock rubble would eventually get clogged with junk and that would make a mess also.
I don't think that the media baskets get water flowing through them that much. More like a tea bag effect.
by the way, I have 3 penguin 330's. Used to have 2 on my 75 reef and just removed one. Live rock does a much better job of bio-filtration. I just use them for mechanical-(clean the pads weekly) and water flow.