Hello
Can you please explain why you need to do that? Please explain the business need behind this.
Activity steps have a single outcome (generally) and that is the default path of the flow when the work item is completed. Do you need an 'additional' outcome? , Then create an event in your custom BO/Class (which I assume that you are already using and have an instance of it in the workflow). Set that event as a terminating event in your task behind the activity step (remember to bind the _EVT_OBJECT form the event to the task). The object should be the same object instance in the task which is of the same BO / Class type of the triggering event).
Now you will see an additional ouctcome in your workflow for that terminating event.
Trigger this event from the code where you want to "Reject' the work item. use FM SAP_WAPI_CREATE_EVENT to trigger the event.
This code can be in an enhancement which gets called in your rejection process.
If this does not makes sense, please elaborate on your rejection functionality, where do you want it to see in the screens and why you are using an activity step instead of a decision step.
Regards,
Modak