There is no easier way in out-of-the-box EA, no. But where there's a will there's a way.
First off, make a copy of your project. The solution I'm describing will destroy its structure.
Second, create an empty package and move the diagram there.
Third, create a VBScript in the Browserscript
group (Tools - Scripting). Call it "Collect Diagram Elements". In the editor, replace the commented-out otDiagram
case with the following:
case otDiagram
' Code for when a diagram is selected
dim theDiagram as EA.Diagram
set theDiagram = Repository.GetTreeSelectedObject()
dim dObj as EA.DiagramObject
dim element as EA.Element
for each dObj in theDiagram.DiagramObjects
set element = Repository.GetElementByID(dObj.ElementID)
element.PackageID = theDiagram.PackageID
element.Update()
next
This script runs through all the elements shown in a diagram and moves them to the package the diagram is in.
Run the script by right-clicking the diagram in the package browser and selecting Scripts - Collect Diagram Elements. After the script finishes, you may need to reload the package (right-click the package in the package browser, select Contents - Reload Current Package).
Finally, export the package to XMI (right-click in the package browser, select Import/Export - Export Package to XMI File), and then import it into your target project.