There may be more sophisticated methods but you could Export your Contacts from Outlook as a Comma Separated File and then import that into Outlook Express.
In Outlook go to File, Import and Export, then Export to a file. Choose Comma Separated Values (Windows) and highlight the Contacts folder on the next screen. Next choose where to save the file and give it a name.
In Outlook Express go to File, Import, Other Address Book, then highlight Text File (Comma Seperated Values). Click the Import button and browse to the file you just saved.