You can learn about sif format here, thanks to @rodolforg :
https://synfig-docs-dev.readthedocs.io/en/latest/common/sif_file_format.html
Plugins are just python programs, which are provided the file as argument, you perform operation on the file itself.
You can find more information here:
https://synfig.readthedocs.io/en/latest/plugins.html
Also checkout previously existing plugins like Lottie exporter (in-built) , Plugin by Glax, Joystick Plugin, etc. to get idea of plugins.
As this is going to be based on other input, you need to create a importer plugin and then create a sif file from it.
Importer and Exporter plugins are only supported on development version 1.5.x