Apart from being manually added into the system, where else might Pending Workers come from?

The Pending Worker Self Register process allows workers to register themselves as pending workers in the system. This is done through the use of self-registration forms that are sent to applicants or other potential workers. When the form is completed and returned, Oracle HCM processes the information to create a pending worker record. This allows the workers to have access to the system and complete the onboarding process without the intervention of an HR representative.

https://www.oracle.com/webfolder/technetwork/tutorials/obe/fusionapps/HCM/WorkerRestAPI_CreateWorker/html/index.html