You can use a TypeConverterAttribute with a custom TypeConverter, something like this:

public class Sample
{
    public Sample()
    {
        Ints = new List<int>();
    }

    [TypeConverter(typeof(MyConverter))]
    public List<int> Ints { get; }
}

public class MyConverter : TypeConverter
{
    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(string))
            return "Hello world";

        return base.ConvertTo(context, culture, value, destinationType);
    }
}