Mesh deforming will be a default feature in Spriter 2, which will be a free upgrade for all Spriter Pro owners. (Spriter 2 is in the earliest stages of development, so don;r hold off current projects for it. )
You can use image sapping to swap out images with warped versions.. in combination with careful separation and jointing of images, along sith standard stretching and rotation go get a lot of liveliness in your animations.
Community member bwwd has also made excellent use of the non-official proof of concept feature called "skin mode" to add mesh deforming to his animations. Which is fine if you just want to bake out your animations to sprite sheets or sequential images, or if you are willing to export the deformed skin images as sequential images, then replace the skin sprites in your final animations with the image swapped versions (IF you need to actually support the Spriter version of your animations directly in your game project)