Directly migrating custom objects from IdentityIQ to ISC is not supported. The migration from IIQ to ISC is anything but lift-and-shift. This is because, in IIQ, a custom object stores key-value pairs, configuration data, or reusable logic. ISC does not support the same object model.
There are some strategies available to you to help with this migration. I would start by reviewing this thread here: Guidance on Migrating from IdentityIQ (IIQ) to Identity Security Cloud (ISC) - Identity Security Cloud (ISC) / ISC Discussion and Questions - SailPoint Developer Community
Here’s a video series webinar on this transition from SailPoint: https://community.sailpoint.com/t5/Webinars/How-to-migrate-from-IdentityIQ-to-SailPoint-Identity-Security/ec-p/254791?_gl=1*1djfufk*_gcl_au*MTQ1NDUzNDU1Ni4xNzUyNTE3NDYyLjEzNDc2ODQ4MzAuMTc1MzQ3MDU1OS4xNzUzNDcwNTU5*_ga*MjM2NzUyMzE1LjE3NDQ2NjAxODY.*_ga_CDHGD2GT5X*czE3NTg4MTc2MDAkbzE5NCRnMSR0MTc1ODgxODc5NiRqNSRsMCRoMA..
And an article on migration readiness: Are you ready to migrate from IdentityIQ to Identity Security Cloud? - Compass.