If you mean converting how Hype styles elements to CSS code, there’s no method to do this. Hype applies styles directly to the DOM and often uses several methods/techniques that aren’t even all represented in CSS or browser dependent.
If you need to get specific values out (say to communicate to an engineer) I simply recommend looking in the inspector. You can also copy the element in Hype and paste into a text editor; this will give a DOM representation but is very specific to WebKit and won’t be great for general purpose use. Likewise with an export you can inspect the element using a browser’s developer tools, but the CSS values may not be applicable to all browsers.
On the other workflow side, if you have some CSS you’ve made on your own that you want to apply to Hype elements, you can really put that wherever you want… for example you could have a link to a .css file instead of embedding in the head’s style tag and that might help you be more portable.