Sigma EX 70-200mm Pentax

[robtyr]

Hi, I'm new to Outdoor Photographer. I've a Sigma EX 70-200mm Pentax II /2,8 DG APO Macro and the Sigma APO Tele Converter 1.4x. Unfortunately the converter doesn't work with this lens and the Pentax mount (it does work with any other mount). Does someone know a converter that is working with this lens? I use as a body the Pentax K5. Thanks.