When you open and read new email messages, you can use the Contact Update button in the Mail Editor window to add new contacts for the unknown addresses found in the email.
Clicking on the Contact Update button displays the Contact Update Window as shown below:-

Contact Update Window
To create new contacts from the unknown addresses found in the from, to and cc fields of the email message, the Contact Update window displays the unknown addresses in the list box (1) to the left of the window. You can select each address to create by ticking the address in the list box (1).
Next, you need to select which contact folder will be used to store the new contact address, from the tree view displayed on the right hand side of the window.
Once you have ticked at least one Address and you have selected the contact folder to store the address, you can click on the Create button to add the selected contact Address to the contact folder. You can store address in different folders by repeating the process for individual addresses.
To close the window and return to the Mail editor, click on the Ok button (4).
Further reading from Adding Contacts: