Prepersons stores in the same table as persons - MPI.persons. Thus all constraints should be removed from this table except status field. For prepersons needs will be added:
external_id field of varchar(1000) type - external identifier of preperson in MIS (also can be used as note?)
unidentified field of bool type - needed to specify if person identified or unidentified. Helps to implement separate logic for prepersons, for example, inform user who gets person via API that he recieve preperson (because in Get person API will be used the same schema on response as for the identified persons)