In principle - "yes".
If you turn on "Store Scans" - you'll see that an inventory scan is "JUST" a text file by and large.
You do that here (requires access to the console on the CORE):
... then here ... (click on the picture for full size version)
So you CAN create your own freestyle as long as you respect the format.
So yes - "you CAN" put in anything you want, by and large. You just need to follow the BNF-paths sensibly and then you're fine.
Hand-crafting data (well - or "automated injection") is something that's perfectly fine & acceptable as an advanced use case.
So I turn on Store Scans.
How would i add a monitor to our inventory?
Look at the way the inventory scan file is constructed.
... decide what / how you want to populate those things for a monitor.
I.e. "Name" (you'll want one) ... possibly unique, i.e a serial number? I.e. "BENQ_123456789-B" for instance.
... and so on.