C#: How to extend PropertyInfo?

I have a solution where i get to map out all properties and sub-properties of an object into a Dictionary. Lets say I have something like these objects:

class MyClassA{
    string info;
}
class MyClassB{
    string info;
}
class MyClassC{
    MyClassA a;
    MyClassB b;
    string something;
}
class MyClassD{
    MyClassC c;
}

I created an utility to map out all the propreties so i can get something like:

MyClassD dObject = Something();
Dictionary<string, PropertyInfo> propertyMap = new Dictionary<string, PropertyInfo>();

propertyMap = buildPropertyMap(dObject );

where the string is the path and the PropertyInfo the actual property. The strings on the Map on this example would look like this (pseudo-output):

propertyMap.Keys={
    c;
    c.a;
    c.b;
    c.a.info;
    c.b.info;
    c.something;
}

This is a great way to tell what goes where when reading for example data from an excel file and not from xml-like things, like so:

ExcelTableC:

 -----------------------
1|Ainfo|Binfo|Csomething|
-------------------------
2|value|value|valuevalue|
3|value|value|valuevalue|
 -----------------------

It wall works great. Now thing is, this is all obviously inside a couple of loops and diferent functions (because of the Excel reading process) and i need later to get the key, this is, lets say I have this 'property' and I want the path (dont ask why):

// this method does not exist (pseudo-code)
PropertyInfo.GetPath; //it would return 'c.b.info' for ex. or 'c.a.info'

So i wanted to implement a class that extended PropertyInfo to add my methods.

But doing something like:

public class PropertyField : PropertyInfo
{
    PropertyField parent;
    string path;
    // etc...
}

returns error because PropertyInfo is an abstract class and I would need to implement all inhereted members.

I can add 'abstract' to 'PropertyField' like this:

public abstract class PropertyField : PropertyInfo {}

But then when i try to cast it like this :

private void findProperties(Type objType)
{
    PropertyInfo[] properties = objType.GetProperties();

    for (int i=0; i< properties.Length; i++)
    {
        //PropertyInfo propertyInfo = properties[i];
        PropertyField property = (PropertyField) properties[i];
        //do something with it
    }
}

will return the following error:

System.InvalidCastException: Unable to cast object of type 
'System.Reflection.RuntimePropertyInfo' to type 'App.models.PropertyField'.

So the question is, how do I add these methods? If I cant inherit what can i do?