You can import a contact list to Horde.
You can export the contact list that you have stored in Horde.
You can import a contact list to Round Cube by going to Address Book at the top, then click the "Import contacts" icon.
You can export the contact list that you have stored in Round Cube by going to Address Book at the top, then click the "Export contacts in vCard format" icon.
You can select more than 1 contact at a time by either holding down "control" or "shift" when clicking on a contact in RoundCube. When you do this, you can export multiple contacts.
In order to import and export your address book, you will need to install the SquirrelMail Address Book Import-Export plugin.