You don’t need cloud Lightroom (what they used to call CC) to do this. Adobe Portfolio imports photos from Lightroom Photos on Adobe Creative Cloud servers. You can use Lightroom Classic to upload to Lightroom Photos by syncing any number of photos from a Classic catalog to Lightroom Photos in the cloud. You don’t have to use any cloud Lightroom apps, just Classic.
If you have a specific set of multiple photos you want to add to Portfolio, create a Collection of it in Lightroom Classic, because you can sync a collection to Lightroom Photos in one click. That will also make that set of photos easier to find when you choose to import from Lightroom when adding a page element (e.g. a gallery) in Portfolio.
One further question: if I upload to the cloud from Lightroom Classic does it store copies of the photos on my PC as Lightroom CC does?
It actually works the other way, because if you are using Lightroom Classic, storage of originals is all on your computer. If you want to upload photos from Lightroom Classic to Lightroom Photos in the cloud, the photos must first be part of the current Lightroom Classic catalog.
Also, cloud Lightroom only stores photos on your computer temporarily, since with cloud Lightroom, storage of originals is in Lightroom Photos on cloud servers. The only way cloud Lightroom will permanenly keep original photos on your computer is if you turned on the preference setting that does that.