You could override ToString(), like this

public class MinorFrameFormatDefinition
{
    [Description("Word Number")]
    public int WordNumber { get; set; }

    [Description("Number of Bits")]
    public int NumberOfBits { get; set; }

    public override string ToString()
    {
        return "hello world";
    }
}

Or if you don't want to change the class, you can also define a TypeConverter on it:

[TypeConverter(typeof(MyTypeConverter))]
public class MinorFrameFormatDefinition
{
    [Description("Word Number")]
    public int WordNumber { get; set; }

    [Description("Number of Bits")]
    public int NumberOfBits { get; set; }
}

public class MyTypeConverter : 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);
    }
}