When viewing knowledge base articles, service requests or problem reports, partners will have an additional option in the content menu items along the top.
The Save As button (marked in orange since it's only available to partner admins) allows you to save that item into an existing Content package in the Partner Content area. Any content in the client area can be copied including content that originated from the Content area.
Everything is copied over to the Content area except for any file attachments since the Content area does not support attachments. Items are always added and never replaced.