Alternatively you van create your own sif files with an external application. They are written in xml format so it is easy to write your own ones. There is not public documentation for the sif file format other than the savecanvas.cpp and loadcanvas.cpp files (the code that converts the Synfig memory data to a xml file and vice versa).
There are examples of people doing this to create some kind of special effects like explosions, etc.