You can, however, hyperlink one presentation to another.

Linking to a presentation that is in portrait format will work. Be sure that you do not have PowerPoint set to end with black slide. After the last slide in portrait format, you will return to the previous presentation, presumably in landscape.

But, as JohnWilson pointed out, the projection will not be changing.

I was shown this method in a PowerPoint class and still use hyperlinking presentations a lot. Usually creating an agenda for meeting that links to all the speakers' presentations for seamless meetings. The instructor had linked to a portrait presentations to make a point. It was generally considered an interesting but essentially useless example.

You can always put portrait formatted material in landscape PowerPoint slides. The medium is essentially landscape, though.