To get any key, first you need to what is sent out when you press the key. If your scanner can read most anything, this is in the handbook somewhere. In HHP IT3800 it is called "Supported interface keys" in Section 12 at the back.
The PLUS key is not listed.
But.... there are others way to get the desired result. Knowing what is sent the IT3800 can take one barcode and convert it to any "key press" string.
This will give you a start I'm looking inot it further now.
DougP, MCP, A+