Obviously, you could make plugins or edit Jekyll to work with them. But, Stasis, a new-ish static site generator uses these, as well as other templating engines and pre-processors, out of the box and is a breeze to use.
I don’t regret using Stasis. I think it’s great. But, I greatly over estimated the difficulty, and complexity, of making Jekyll plugins. It couldn’t be simpler. Here are a couple (Sass, CoffeeScript) that I’m using for the new version of this site.
There are reasons to use Stasis over Jekyll, but, support for Haml, Sass and CoffeeScript, as I mentioned before, isn’t one of them. Just do it yourself!