1) Access Open-Xchange webmail 

2) Click on the Address Book tab

3) Select which contact list that need export, and click on the button beside as screen shot

4) Click on the Export button

5) Export out as vCard format (.vcf)

6) Log out and re-log in webmail again, now select Horde webmail

7) Click on the Address Book tab then click on the Import/Export button 

7) Select the vCard as source file. Click on the Choose file under Import Address Book, then select the .vcf file that export

8) Click NEXT button and address book will be import to Horde webmail