Workflow on transactions

Hmm, been trying to see if it's possible to maybe add items onto a transaction.. specifically set some conditions on invoices which gets them to enter workflow, then add a button "promotion" which when clicked gets the workflow using the add action to add a specific inventory item, or maybe a discount, etc to the transaction. Is this going to be doable? I have had a few attempts but don't have it working.. Thanks!

I am thinking maybe the kind of action I want.. something like "add line item" is not an action that can be done? It needs to be a custom workflow action script, "add line item"? any pointers/advise appreciated