Hi Anand,
how are you calling the EJB ? as a mapping function?
For steps which are not driven by a user through a UI, the process server uses the service user SAP_BPM_Service. This user is already pre-configured. However, if any specific roles/athorizations are required, you can configure this user as :
Configuring BPM Users - Configuring Business Process Management - SAP Library
As a default user, the SAP_BPM_Service user is used if no other user has been specified.
Alternatively, you can also transfer the named user of the prior process step or event via the Principal Propagation technique Principal Propagation - Business Process Management Security Guide - SAP Library
-Abhijeet