I'm pretty sure you can't do something before Awake - however, how about you get the prefab - change the variable in there and then do an Instantiate on it? That would mean the value was set before you instantiated the copy.

Given that instantiate just makes a copy of a GameObject - you have the GameObject that is your prefab, you can just change that in the code on the line before the Instantiate and then Instantiate will make a copy using the updated value.

To expand on my previous answer (which I'm doing because this does solve the general problem of setting a variable that needs to be used in Awake or changed before it is called for some reason).

Imagine you have a prefab that contains a reference to another object that is used in Awake:

 using UnityEngine;
 using System.Collections;
 
 public class DoSomethingInAwake : MonoBehaviour {

      public GameObject otherObject = null;
 
      void Awake ()
      {
          print (otherObject.name);
     
      }
 }
 

That is attached to a prefab - let's say it has null in the otherObject. Clearly this raises an exception when it is instantiated.

The following code could be used to initialise the value of otherObject in advance:

 public class InstantiateSomething : MonoBehaviour {
 
     public Transform prefab;
 
     // Use this for initialization
     void Start ()
     {
         //add these lines to update the value before instantiation
         var dsia = prefab.GetComponent<DoSomethingInAwake> ();
         dsia.otherObject = GameObject.Find ("Main Camera");
         //instantiate the updated prefab
   
         GameObject.Instantiate (prefab);
         //prints "Main Camera"
     
     }
 }

In your case you would change the GetComponent to return the path agent and set m_pathManager to your current path manager.