It seems like there are now two formats used in synfig, “.sfg” and “.sifz”.
I am looking for documentation on these formats so I can write a generic C++ library that can import them.
The following seems to be the extent of the documentation:
[]External plugin to write .sif]
[]SIF format specification]
bioalchemist welcome to the forum,
In fact it’s three format, you mess .sif who is non compressed sifz (aka readable xml)
Oups, … actually nothings was done in that direction … but, “sif” is “readable” … Haramanai have done some javascript work to convert sif to html5 …
Digging in that way, if you can draft (wiki : dev ) the format could be very useful for the community … ! 
What kind of conversion are you thinking about ?
Who wrote designed the format? There’s got to be at least a reference document, school drawingboard, napkin, etc.