I think you are talking about folder items. You can put more than 25 items in one folder. But only twenty five will show on one page. In order to get to items past the 25th, you have to click on the second page, a link that appears automatically when the 25 item limit per page is reached.
This feature does not depend on having a premium membership. But, as you can see in "Compare Free and Paid Membership Benefits" , there is a huge difference in total items you can have in your portfolio. Depending on how many poems and other items you want to create, having at least an upgraded membership is a very good idea.

You can make folders in the same way that you create a static item. Go to "My Portfolio" "Add Item" "Folder". You have to give it a name and an intro, just like any item. There has to be a short blurb in the text area. Then, after saving the folder, you can put your poems into it. That's easily done by opening your item (click 'edit" in the slim black car at the top) and then between the title and intro, you will find a drop down menu which will offer to put the poem into your portfolio, or into the folder.

I hope this all made sense and answered your question.