Code:
<GameData>
<Civilization_FreeUnits>
<Row>
<CivilizationType>CIVILIZATION_AMERICA</CivilizationType>
<UnitClassType>UNITCLASS_WORKER</UnitClassType>
<Count>1</Count>
<UnitAIType>UNITAI_WORKER</UnitAIType>
</Row>
</Civilization_FreeUnits>
</GameData>
As a general rule, first see if the base game files have any light to shed on your request, because the information the base game files have forms the basis of pretty much every single mod out there.