Import/Insert/Copy/Merge .3dm file into layer?

Situation:

I have a .3dm file named. I want to import/insert/copy/merge (pick your verb) that file including its layer structure, geometry, etc into a specified layer of a model. How can this be done, preferably without opening the file to be imported/inserted/copied/merged? Compared to Worksession I want to have a single file.

Example:

Starting model:
Before.3dm (1.7 MB)
Before

.3dm file to be imported/inserted/copied/merged
Stuff.3dm (1.7 MB)
Stuff

After model:
After.3dm (1.7 MB)
After