And YES your Sigma Lens will fit the Sigma extender!
So the Canon lens won't fit the Sigma extender, but will the Sigma lens fit the Canon extender? The Sigma 150-600 C is on my wish list and I had been thinking I'd also like to get the extender. I have tentatively identified the Sigma because it's a bit cheaper, but it might be worth a little extra for an extender that works with more of my lenses. Of course, I would probably only really use it with the 150-600. If I needed more reach than my 70-300 provides I'd probably switch lenses before adding and extender. Of course all predicated on the assumption that I get the 150-600!