Is there any way to copy diagrams into different projects?

in the model explorer view, you can copy a model, then open another star uml with your other project and in the model explorer view, you can now paste the model you have copied.

You can export / import fragments, which are items in the model (in file menu, version 2.5)

You can copy-paste elements only one-by-one. But you should work with Model Elements. Review please, first method works for me, but the second