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 Conacts link on the left. Click on the import icon
Select the file downloaded from Horde and click Import to start the import.
