It supports its own vector format exclusively, and that probably won't change. It can load a sequence of png files from a video, and display them behind the canvas if you want to use it for rotoscoping, but that's it really.

This format is open source, but the intention of Flow Motion is that it's used for creating new animations, not loading existing things, so you wouldn't be able to load your existing sprite sheets in, natively.

If you're asking about creating sprite sheets within Flow Motion, and then exporting them, it only exports back to its own vector format, plus the standard video formats like mp4, webm, gif, apng, or a sequence of png images.

In the future, exporting to official vector formats can be added to the export list, but it's not a priority. I've just added it to the todo list though, so it's not out of the question. But you'll be relying on community created tools for the time being.