is all syntax definitions written for Fragaria or just conventered from some other format?
The syntax definitions are simple dictionaries rendered as property lists.
The structure (key names etc) is unique to Fragaria.
That said, the syntax definitions are simple and transforming them in to JSON or YAML would be a simple matter of loading up the plist and dumping it out using the likes of JSONKit or YAMLKit.
Anyone got a pre made YAML syntax definition though?
YAML is a markup scheme not a language with a pre-defined syntax.
Most of Fragaria's highlighting is keyword based which might make YAML highlighting more difficult as you will need to use regex's to identity the keys etc.
Some of the other forks might have better regex support for colouring.