Here's an article on what food would be available in a Nuclear Winter.
It prompts a lot of questions and the thing about the Twilight War is that it always postulated a 'friendly nuclear war' that wasn't overly destructive. In fact every T2K campaign I've been in on reflection the damage to the fabric of the world probably wasn't as bad as the Second World War and never as bad as The First World War (there's places in northern France that are so contaminated by chemicals from gasses and explosives from The First World War that they kill 95% of plants and are so toxic they probably can't be entered for another 700 years by unprotected people)
However a true Nuclear Winter is a really terrifying setting. A general exchange would probably stop most sunlight reaching the surface of the earth and survivors have about two to three years to get to the equatorial areas and set up enough infrastructure to exist. A campaign set on this isn't just getting back to friendly territory but transiting a whole section of a global hemisphere.
It also is a lot harsher in every respect. Everyone left behind is going to starve or die of exposure. Food supplies are going to rapidly exhausted in that short window of transit because the very next crop is going to fail after the exchange, and its going to fail on a global scale and keep on failing. Players are not going to be able to comparably comfortably wait out the winter months in a canton but instead are going to have to keep moving as the cold starts to set in around the polar latitudes and then move. Endless darkness from the nuclear clouds will make the going tough. Most people will simply psychologically fail and die, unable to move. In fact most surviving people may be even unaware that they have to move towards the equator and this will doom them.
It definitely makes for a different game in every sense. The article says that a general exchange between just India and Pakistan is probably enough to trigger the Nuclear Winter, an exchange between the WTO and NATO is more than enough to do so, even one that only uses a fraction of the combined arsenal.
The biggest problem once the players get moving may just be starting. A general mass movement in an equatorial direction will overwhelm every society on the planet, both those moving and those already at the equators. Players may well need to fight the whole way and the fight for space once they're there and finally fight again against others arriving.