Add number as makes it a lot easier for you to add new contacts to your phonebook. 3 actions are required to add a new contact:

1. Select a Call Log entry or SMS message which you want to add to the Address Book.
2. Press the Menu key, select Add Number As item. Depending on application settings you may be asked to select a phone number type (e.g. Work, Home, Mobile).
3. Fill contact info as usual and select Save from menu to save contact

Minimum requirements are blackberry OS 4.2.1