No need to rebuild your entire contact list, just import it from your current application and format it to your address book in OS.

In order to import your clients, you will first need to Export them from current application.

This tutorial covers how to Export your Contacts from Apple Mail.

1. Open your Address Book and select the contacts that you want to export
2. Select “File” at the top of your screen.
3. Select “Export” from the list
4. Select ” Export Group vCard”
5. In the “Save As” field type a name for the file.
6. In the “Where” field select a destination.
7. Click Save.

Once you have your vCard export file, you can now Import your Contacts into MYbusinessOS!

See this article on Importing you Contacts