How to overwrite/set a (newly) variant-describing feature in Hybris

I want to assing new values to a feature of a (variant) product in hybris. The features (qualifiers) and their values are defined in a textfile. The feature is already part of this product classification but it is currently neither set at the variant nor at the base product. Hence both times Null is returned when I try to fetch it:

feature = getFeatureFromProduct(product, qualifier)
if (! feature) {
    log.info "$qualifier does not exist for $product.code"
}

base = ((VariantProductModel) product).getBaseProduct()
log.info "Base: $base.code"
baseFeature = getFeatureFromProduct(base, attribute.qualifier)
if (! baseFeature) {
    log.info "$qualifier does not exist for $product.code"
}

That's the method to get the feature:

static ProductFeatureModel getFeatureFromProduct(ProductModel product, String qualifier){
    return product.getFeatures()
        .stream()
        .filter {feature -> feature.getQualifier() == qualifier}
        .findFirst()
        .orElse(null)
}

So I have to do one or two things:

1. Create an new ProductFeatureModel based on the qualifier and value.
2. Assing it to the product. (and maybe register somewhere that the feature may now be different for variants, so it's variant describing - I don't know the exact terminology for that)

How would I do that in groovy / using the hybris Java API?