The property grid uses TypeDescriptor to gets its converter information. So for example, if you have this class:

public class MyClass
{
    public int? NullableInteger { get; set; }
}

With this code:

// get first property's type converter
var cv = TypeDescriptor.GetProperties(typeof(MyClass))[0].Converter;

You will get the TypeConverter for the NullableInteger property, which is already NullableConverter.

So, you don't have to declare it manually.