Yes, you can add offline investments on your Portfolio Dashboard. To add an offline investment, click the purple "Add Investment" button at the top of the Table of Investments.
This opens a modal where you can enter more information about the investment.
Click on the "Bulk Upload" tab to download a CSV template from AngelList that you can use to organize your offline investments. Once you've filled out the spreadsheet with the offline investments you would like to show on your AngelList Portfolio Dashboard, upload the updated CSV file to import your investment info.