I think there are a few ways you could exclude things from a build process, but it might only be certain levels that you can exclude. You can of course put it in a layer but whatever is in your content browser will be built with it. The players just won’t be able to see it like putting it in a layer, etc.
If you don’t want it to be built at all with the project, you can of course do as you say, migrate from another project.
Hopefully that helps!
NOTE: The settings in the image above can be found in your Project Settings