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 Windows Live Mail.

1. Choose “Contacts” from the list in the bottom left corner
2. From the list on the top ribbon click “Export”
3. Click Comma Separated Values.
4. In the “Save exported file as” window, click Browse, select a destination folder for the export and type a name for the file in the “File name” field.
5. Click Next
6. Select the fields that you want to export by clicking the box next to the field name. Make sure that any relevant fields are ticked, as only the marked ones will be exported.
7. Click Finish.

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

See this article on Importing you Contacts