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 the Blackberry Address Book.

1. From Organizer select your Address Book
2. Click on Setup.
3. Select “ASCII Importer/Exporter”.
4. Click on Next.
5. Select “One way sync from device”.
6. Click on Options.
7. Select Comma.
8. Click on Browse and select a location to save the file.
9. Enter a name for the file in the “file name” field and add .csv to it  (e.g.  contacts.csv).
10. Click Open and when a message appears saying, “This file does not exist.  Create this file?” click Yes.
11. Click Next .
12. When the message, “Congratulations.  You have successfully configured the product” appears click Finish.
13. Click on OK.
14. You will be returned to the Organizer screen.  Make sure the Address Book is checked and click on Sync to generate the file.
15. The file will be saved in the location previously selected.

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

See this article on Importing you Contacts