In the 1&1 SMS Manager you will need to go to the "Address Book", at the bottom if the screen you will see an "Import" button. Click on this to access the import options.
You can import formatted text files for you contacts, most e-mail clients will have a facility to export the contact list as plain text or .csv formatted file.
Save this file on your local computer then use the "Browse" button to locate it. When you have selected the file click on the "Upload Contacts" button.
Your imported contacts will then be shown in the contact list.