Technically yes, but probably not with practical results.
You can take haxe code and integrate it with Stencyl by either using extensions, 'code mode' or 'freeform mode' editing, or even replacing parts of the Stencyl engine. You couldn't edit your code using the blocks (design mode) or have Stencyl automatically segment your code into behaviors.
Unless you already know your way around the Stencyl engine and a bit of its internal workings then it would take less time to just start over and use your existing code as a guide to refer to.