Sunfish allow plugin creation too add more functionality through service types.
For details see Sunfish Text Plugin solution.
Create a new C# library project, the assembly name should start with "sf-" and use Sunfish.exe as reference. If the Sunfish reference is set as project, also copy the $sunfish directory
If the resulting assemlby is not a .dll and not starts with sf- Sunfish will not load it.
Also is hight recomendable to set the startup program on debug as the Sunfish.exe inside bin/debug folder.
Place the plugin on the same path ans sunfish.exe
Newtonsoft's Json.NET is available for plugins.