If Covve is linked to Gmail or Outlook / Exchange
To use your Yahoo contacts with Covve you can export them from Yahoo and import them into the address book that you have linked with Covve (gMail or Outlook). These will synchronized with Covve:
1) Navigate to your Yahoo contacts
2) Click on "Actions" and "Export"
3) Select "vCard single file" and click on "Export"
Your Yahoo contacts are now saved on your computer.
Instructions on how to import your contacts can be found:
a) Here for Gmail
b) Here for Exchange/Outlook
c) Here for Office365
If Covve is linked to your iPhone
Alternatively, if Covve is linked to your iPhone you can add Yahoo to your iPhone contacts and this will in turn synchronize them with Covve.