How to deep copy a class without marking it as Serializable

Given the following class:

class A
{
    public List<B> ListB;

    // etc...
}

where B is another class that may inherit/contain some other classes.

Given this scenario:

1. A is a large class and contains many reference types
2. I cannot mark B as [Serializable] as I don't have access to source code of B

The following methods to perform deep copying do not work:

1. I cannot use ICloneable or MemberwiseClone as class A contains many reference types
2. I cannot write a copy constructor for A, as the class is large and continuously being added to, and contains classes (like B) that cannot be deep copied
3. I cannot use serialization as I cannot mark a contained class (like B, where no source code available) as [Serializable]

How can I deep copy class A?