In your WordPress install go to Tools > Export. In "Choose what to export" select either Post or Pages then click "Download Export File" to download XML into your computer.

Do not select "All content" since you will get an error saying there are duplicate messages when you try to import it to Porfolio. 

Create a new Snippet group in Porfolio and click "Import CSV" button 

Or open a group already created and click CSV icon.

In the new window click “Import XML" button and select the XML file from your computer.

After you import XML file all your WordPress post or Pages will appear with their respective link preview on the left side. On the right you will be able to delete messages you don't want to add to the group by clicking trash icon. After deciding which messages you want to add click ''Save" button. The messages will be imported to the snippet group so you can use them in the future.