In the following documents,

https://access.redhat.com/documentation/en-us/red_hat_process_automation_manager/7.3/html/process_designer_business_process_model_and_notation_bpmn2_reference_guide/bpmn-tasks-overview-con

it's listed the following predefoned custom tasks

• Rest service tasks: Used to invoke a remote RESTful service
• Email service tasks: Used to send an email
• Log service tasks: Used to log a message
• WebService service tasks: Used to invoke a remote WebService call
• DecisionTask tasks: Used to execute a DMN diagrram

but detailed steps about how to use it is not documented.
Regarding BPMS6.x, there is the following documents

https://access.redhat.com/documentation/en-us/red_hat_jboss_bpm_suite/6.3/html/user_guide/appe_service_tasks

Customer requests to provide equivalent document for RHPAM7.