You can add people automatically to your contacts when you reply to received email. Enable the plugin Automatic Contacts by going to EditPlugins; click on "Automatic Contacts" so that it shows a check mark. Then go to EditPreferencesContactsAutomatic Contacts and enable the Create address book entries when sending emails option and select an address book in the drop-down field below that will receive the new contacts.

From now on, contacts will be automatically created for senders of received mail when you reply to that mail.