XMI import/export is currently only supported for an entire model (including any profile implementations etc. that have been added to the model). From the context menu of an individual package, XMI export is also available via the "Tools" sub-menu. However, counter-intuitively, this option still exports the entire model, not just the selected package. Other - much cheaper - modeling tools usually provide package-level XMI import/export, so I think PTC Integrity Modeler should do so as well.