Migrating this data is optional.
An OCLC non-bibliographic data analyst will work with you.
OLIB Loan data can be obtained for items that are currently checked out (active loans).
This information is then added to the exported OLIB loans data file.
Find instructions, a file template, and a sample record for formatting and sending your loan data to OCLC:
https://help-nl.oclc.org/Librarian_Toolbox/Circulation_data_files