Make a sub microflow that you feed the string into. In the submicroflow you have a series of gateways, each checking for a different enumeration value (aka the string) like 'Ferrari'.
Then have the microflow return an enumeration using your specific enum and have the gateways return their appropriate enum value (Cars.enumeration.Ferrari) if they return true.
You could do it in Java.
Input = String (should be equal to Enum caption!)
Output = Enumerator
Code example (Enum = KavelKavelTypeEnum, values = Prijskavel, Volumekavel)
if (KavelKavelTypeEnum.Prijskavel.getCaption() == "MeegegevenString") {
return KavelKavelTypeEnum.Prijskavel;
} else if (KavelKavelTypeEnum.Volumekavel.getCaption() == "MeegegevenString") {
return KavelKavelTypeEnum.Volumekavel;
} else {
return null;
}