It should be possible to accept an identifier such as an SOR ID or Enterprise ID from an OIS and map it to the manager_co_person_id or sponsor_co_person_id foreign keys. There are two possible approaches:

1. Write an inbound Data Filter that looks for an identifier in the inbound record and maps it to either the manager_co_person_id or sponsor_co_person_id foreign keys. The inbound identifier will probably need to be conveyed as an AdHoc Attribute, and the Org Identity data model would need to defined the _id foreign keys so the Data Filter can convey the correct values to the Pipeline.
2. Add new manager_identifier and sponsor_identifier fields to OrgIdentity, that accept a string rather than a foreign key (which makes sense in the stricter data model, since how would an External Identity Source know what the co_person_id is?). Pipelines would then map the identifier to the co_person_id foreign key, possibly with an Identifier Type configuration.