What you are trying to do is not possible, the editor and the player are not connected. The first is to edit/create a scene and export it, to be played by the later…
Imagine your question translated in image editor paradigm (let’s say Photoshop): you ask that when you add an object by a macro in Photoshop you want the resulting jpeg to be updated when shown in a (whatever) external viewer like a browser…
If you want to add an object to the scene exported in the player running, you have to use normal three.js API also exported into your scene, like:
Example a script added on the scene object:
[Scene / script name]
function start (event) {
var myObject = .... from loader or new instance ...
scene.add( myObject )
}