Feed GUID or externalId to contact field or custom data object

Summary: We require the GUID or externalID to be passed into a contact field or custom data object within Eloqua for tracking and reference purposes.

Content (please ensure you mask any confidential information):

Hi,

We’re exploring the possibility of studying GUIDs across our marketing channels. Would it be possible to feed Eloqua’s backend value for externalID into a contact field or a custom data object (CDO)?

I’ve reviewed the REST API’s Retrieve Visitors endpoint, which includes the visitor’s externalId. Is there a way to capture and map this value into a contact field or a custom object in Eloqua?