Extra zoom + macro lens advice for Canon Eos 350d (1 Viewer)

I have recently purchased a Canon Eos 350d and a Sigma 135-400mm lens which I intend to use for bird photography. Whilst there is no denying that both camera and lens are impressive if the subject is relatively near, this set-up seems limited for distance shots.

I was wondering if is there is a decent conversion lens out there that would add zoom without affecting the auto-focus function and also wouldn't break the bank.

In addition could advise me on a decent macro-lens for detailed insect photography.

Thanks