Reltio Salesforce connector doesn't have the capability at the moment to map the Salesforce object fields that are External ID fields.
If your organization also requires or can benefit from this enhancement, please vote on this enhancement request (MDM-I-1750) so that Reltio can evaluate and incorporate this feature into a future release of the Salesforce connector:
Vote Here!
If you do not have access to AHA! to vote on the enhancement you may request it from your Reltio Customer Success Manager.
Thanks,Brian