You can use your horde webmail interface to manage email contacts between your accounts.
In this video you can learn how to import a .csv, or other file format, containing contacts that you exported from another email account/service.
You can also export your contacts from your horde webmail interface, to be imported to other email accounts/interfaces:
Manually Add/Delete/Modify Contacts
You can also add contacts manually, as well as modify them, view them, and delete them. This next video demonstrates those options: