Just thinking out loud here, but is it possible to trick the ecu to run a little bit richer. I did this before with a car that ran rich. I put a resistor on the air temperature sensor so it thought it was very hot out so it trimmed the fuel back a little.

[2015 Z Force 800 53/Trail EPS]

I would think that if the data from the O2 sensors told the ECU that it was lean, it would increase the injector's pulse width to richen it. It's just a millivolt  reading but I was told that the mapping range isn't very wide so it may not be enough to accomplish what you want without a programmer.

I was thinking about making it think it was sub zero. I  don't think I need too much more fuel, just a little bit.

Is it possible...yes.  The problem becomes when you change altitude that the reading is off also.  Would it work?  Most likely.  Is it the best way to go about it? Nope. Im not saying dont do it but If you want a system that ACTUALLY fixes the problem then there are a could put there that do it and do it right.  EJK and Power Commander are the 2 that come to mind.  EJK is the "simple and cheap" version and Power Commander is the "most accurate and tune-able version.