Horde
You can import a contact list to Horde. Go to the Address Book icon at the top and click the Import/Export icon at the top. Finally, select the type under "Import Address Book step 1" and click the Next button. On the "Import Step 2" page, review the information and click the Next button. On the "Import Step 3" page, confirm that you have at least two field names under "Matching fields" and click the Next button.
You can export the contact list that you have stored in Horde. Go to the Address Book icon at the top and click the Import/Export icon at the top. Finally, select the type under "Export Address Book" and click the Export button.
Round Cube
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.
SquirrelMail
In order to import and export your address book, you will need to install the SquirrelMail Address Book Import-Export plugin.