Hi @primrosem and welcome to the Forum!
As it stands, it is only possible to add projects one by one to a Portfolio. It is not possible to add all of a Team’s projects at once. You can learn more about Portfolios in the following Guide article: Portfolios
There is an existing thread in the #productfeedback requesting this functionality: Portfolio: Add All of a Teams' Projects at Once - #5 by Christopher_Milburn. In case you haven’t yet, I would suggest you to upvote it!
Have a great Wednesday @primrosem