Export your contacts from your existing contact manager using a .csv file format.
Import the .csv file into Postbox.
After the import is completed and the newly imported Address book is in the correct format, select one contact and use the control-a or command-a to highlight all of the contacts. Then simply drag the highlighted list into the Postbox default "Personal Address Book".
Then delete the imported file as it is no longer necessary.
Note: there is a 26 column limit for .csv files. Please use a spreadsheet application to trim any extra columns before importing into the Postbox Address Book.