Unlike on-prem ePO, there's no option to check in a custom package into ePO - SaaS.

In the event where a custom package created using the ENS Package Designer is required to be deployed through ePO - SaaS, contact Technical Support for assistance by raising a Service Request.

Make sure that the exact requirements are provided so that the custom package is created for you by Technical Support and then gets added to your ePO - SaaS in the backend.