Hello,
"I want to trigger notification workitem from custom program into SAP Inbox of all users"
That's easy, make it a General Task and don't assign any (responsible) agents.
But why on Earth would anybody want to do this? The workitem should be sent to the person(s) who can do the work required, not to anybody else.
I would double-check those requirements. Your users are depending on you to give them good advice.
regards
Rick Bakker