You can try inserting something like this in your damage formular for Freezing Breath:
'if (b.isStateAffected(1)) b.addState(2)'
where 1 is the State ID of Wet and 2 the State ID of Frozen.
Then you can add an effect to your Freezing Breath that would remove the state Wet, just like you do with a Cure Skill/Item.
Another way would be giving everyone a 100% resistance against Frozen, but having the Wet state reduce this resistnace to 0.
Lastly, since you are using Yanfly Plugins, his Action Sequence Pack allows Skills to have an if-Condition, where you could make the Wet State a condition forapllying the Frozen State in your Freezing Breath notetag.
EDIT: If you use the first solution, to have Freezing Breath inflict damage add ; before typing your damage formular as usual:
'if (b.isStateAffected(1)) b.addState(2); a.mat * 6 - b.mdf * 2'
for example.