I could look into figuring this out, but I have a huge question. Why? Why do you need this?
Also, if you want to be able to make a project do what something in the snap ui can already do, just look through the source code. You will most likely find the solution. The best way is to search for the string of text that appears in the ui.
why import a scene? it barely interacts with the rest of the project, what difference would it have in your case from just opening the other project?
i don't know any way scenes can be imported without js, i feel like you should either load a sprite using the buggy scisnap primitives, or parse an xml file to create rings, costumes, and sounds.
if you want to use js you can probably just use the browser debugger to step through what happens when adding a scene, although it would probably break the ui on the project page like the sprite snap primitive