Move unassigned story to an assignment (for InCopy)

I want to add a stroy to a new assignment with javascript.

I found out how to create an assignment, and how to make a icml and icma document

However it comes in unassigned incopy items, and I can't find a way to place it into my new assignment.

To create an assignment I use:
var myAssignment = myDoc.assignments.add(...);

And to create an icml file I use:
myStory.exportFile(ExportFormat.INCOPY_MARKUP, File(myFileIcml));

Both work, but now I need a way to place the icml into myAssignment. How can this be done?

Thanks!