Being somewhat new at this, I'm going to give it a go. This is my best guess based on the info already there. Just add into the following areas:
<TraitModifiers>
<Row>
<TraitType>TRAIT_CIVILIZATION_HARDY</TraitType>
<ModifierId>TRAIT_CANADA_SNOW_FOOD</ModifierId>
</Row>
<Row>
<TraitType>TRAIT_CIVILIZATION_HARDY</TraitType>
<ModifierId>TRAIT_CANADA_SNOW_PRODUCTION</ModifierId>
</Row>
</TraitModifiers>
<Modifiers>
<Row>
<ModifierId>TRAIT_CANADA_SNOW_FOOD</ModifierId>
<ModifierType>MODIFIER_PLAYER_ADJUST_PLOT_YIELD</ModifierType>
<SubjectRequirementSetId>PLOT_HAS_SNOW_REQUIREMENTS</SubjectRequirementSetId>
</Row>
<Row>
<ModifierId>TRAIT_CANADA_SNOW_PRODUCTION</ModifierId>
<ModifierType>MODIFIER_PLAYER_ADJUST_PLOT_YIELD</ModifierType>
<SubjectRequirementSetId>PLOT_HAS_SNOW_REQUIREMENTS</SubjectRequirementSetId>
</Row>
</Modifiers>
<ModifierArguments>
<Row>
<ModifierId>TRAIT_CANADA_SNOW_FOOD</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_FOOD</Value>
</Row>
<Row>
<ModifierId>TRAIT_CANADA_SNOW_FOOD</ModifierId>
<Name>Amount</Name>
<Value>2</Value>
</Row>
<Row>
<ModifierId>TRAIT_CANADA_SNOW_PRODUCTION</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_PRODUCTION</Value>
</Row>
<Row>
<ModifierId>TRAIT_CANADA_SNOW_PRODUCTION</ModifierId>
<Name>Amount</Name>
<Value>2</Value>
</Row>
</ModifierArguments>
This should add it in the the trait already there. Or it could not. Still new at this.