You can assign the right to user using WPS Admin Console like this

You will have to give
Can Run As User rights to the user so that he can impersonate other user

Can Run As User rights to the user so that he can impersonate other user

Name: logout.explicit.filterchain
Value:com.ibm.wps.auth.impersonation.impl.ImpersonationLogoutFilter

Name : jndi.com.ibm.portal.portlet.service.impersonation.ImpersonationService
Value: com.ibm.wps.portletservice.impersonation.impl.ImpersonationServiceImpl
ImpersonationService provided by portal for impersonating. 