Yes, you can (at least technically) add to SpecificContent - it’s a dictionary of string, object.
myTransaction.SpecificContent("myNewEntry") = "this is a test"
From that point you can access it just as if it was there from the start.
Do note that you need to pass your QueueItem as an In/Out argument for that to work and importantly that these additions will not be reflected back to the Queue itself (so no visibility of it from Orchestrator).
That was a great question though, I can definitely see some good uses for it, thanks!