I would like to change the essence needed to build an ark from 800 to 500, and modify the slave Drivers of cravers from -10 Approval per enslaved population to -5 Approval per enslaved population.

is it possible? I could not find the file related to these data. 

Hey a3a3311o!

The current cost of the Vodyani Ark is based on its hull, and is equal to the maximum of the Essence stock:

<CustomCost Instant="true" ResourceName="EmpireLifeforce">Property(StockLocation,@'../ClassEmpire',MaximumEmpireLifeforceStock)</CustomCost>

You can modify this formula by overriding the HullDefinition HullLarge00Vampirilis.

Look at HullDefinitions[Vampirilis].xml in the game's files for more info.

For the enslaved population maluses, you'll have to look at SimulationDescriptors[Population].xml:

    <SimulationDescriptor Name="PopulationParasitedPlanet" Type="PopulationParasite">
        <Modifier TargetProperty="ParasitedPopulation" Operation="Addition" Value="$(PopulationCount)" Path="../ClassColonizedPlanet"/>
        <BinaryModifier TargetProperty="Happiness"     Operation="Subtraction" Left="10" BinaryOperation="Multiplication" Right="$(PopulationCount)" Path="../ClassColonizedStarSystem"/>

        <Modifier TargetProperty="BonusPopulationFIDSPercent"     Operation="Addition" Value="1" Path="ClassPopulationPlanet"/>
    </SimulationDescriptor>

Override this SimulationDescriptor, and you can change any of its effects on the population.

Thank you so much. You helped me a lot.