Hi Peter,
First, let me make sure you are clear on what import will do for you.
When you import you have two options: you can import a complete package tree, or you can import a complete model. So, if you have only the (default) model root in your project, the latter course is the most likely one for you.
If you select and right-click a model root node in the Project Browser, the context menu will allow to import a model, overwriting the one you selected.
If you select and right-click a (non-root) package, the context menu will allow you to import at that level.
So far so good.
As far as importing a second model, complete with a root node that is a 'sibling' of the one you have now, that should be easy.
First, create another root node. Right-click the current root node, and from the context menu choose New Model (Root Node). This will give you another root node. Now select that node and right-click again. Do a model import from XMI. You should now replace the new root node with your imported model.
David