How do I map new custom object records to existing contacts via services?

I have CDO records incoming through SF integration app, and I need to map them to contacts.

BUT when i use CDO linking settings in integration app the result is that when a contact is not found, a blank new contact is created - I do not want that to happen. What i need is a method of linking/mapping CDO records to contacts without creating new contacts, just "link if the contact exists". I was thinking about custom object record services but to be honest I am not sure how to set this up in there.

I would be grateful for any help.