You can select any of the built-in export formats (.csb, .json, and .lua) in the “Project Settings”. It’s also possible to create a custom exporter addin in C#, but the documentation is scarce.
On custom widgets, I tried to start some discussion on cocos studio addins but didn’t get any responses. In the end I was able to create a “Custom Widget” and add custom fields to its data, but wasn’t able to get it serialising out into the the .csb. You might be able to get it working if you try with lua. Check the documentation here.