Maybe you could look at the type converter mechanism, which you can use from inside the designer.
That would require that you build your own converter inheriting from
System.ComponentModel.TypeConverter
Another solution maybe to add a property from your enum type in your entity class within the one of the custom user code regions (to prevent overriding on next generation) and to build a custom projector to the same class which hydrate your custom property. (
SD.LLBLGen.Pro.ORMSupportClasses.DataProjectorToCustomClass<T>
)
The first solution maybe easier; in either case, you might have to give a look at the SupportClasses source code for a precise direction.