Export Address book from Horde
Open the Organizing node in the menu to the left and click on Address Book. Click the Import/Export button in the top menu. This should open the page in the screen shot. In the Export Address Book section select vCard (3.0) as export format and the address book to export. Click on Export to download you exported address book and save it to your client.
Import address book to Roundcube
Switch to the Roundcube address book by clicking on the Address Book link in the top right. Click on the import icon to open the Import contacts dialog.
Select the file downloaded from Horde and click Import to start the import.