Lookup Relationships Supported for Files Connect External Objects

Until now, Files Connect external objects primarily existed to let users search external data sources. But in Summer ‘15, you can link fields in external objects to standard and custom objects, integrating external data into related lists and other areas throughout Salesforce.

External objects support lookup relationships similar to custom objects. However, because external data doesn’t always contain the 18-character Salesforce IDs required to associate related records, two additional types of relationships are available for external objects: external lookups and indirect lookups.

For details, see External Object Relationships in the Salesforce Help.