Hi @Danny_Williams and welcome to the Forum 
As it stands, it is not possible to add multiple projects to a Portfolio at once, it can only be done one project at a time. But this is definitely a good feedback for future improvement so I have gone ahead and move this thread to the #productfeedback category to allow you and other Forum Members to vote for this feature request to prioritised. i’ve also slightly modified the title of this thread to better reflect your idea and make the most more discoverable; hope you don’t mind