Hi,
Yes, you can (and should) use SAP_WAPI_CREATE_EVENT.
Workflows are all about objects, so you need to think about what kind of object is being approved. This is the object that will 'own' the triggering event, this is the object that the manager will be viewing and approving, and this is the object that should have the status stored against it.
Please don't use Z tables, and please realize that a workitem is more than an email.
Good luck! I recommend you read the 'Practical Workflow for SAP' book before starting. This old blog might also help:
cheers
Paul