When creating records in an Eloqua Custom Data Object, how can I link CDO records to existing Contact records?

Use the Export: Add/ Update task template to create records in an Eloqua Custom Data Object (CDO). To create records, do not map the DataCard External ID field to an attribute. 

Fill the Map Custom Object record to Eloqua Entity checkbox in Advanced Configurations to link the Eloqua CDO record to an existing Contact record. Remember to click save in the top right corner to save your configurations.