How to change the origin of a gameobject?

I am working on some random generation, and before you laugh at my script, it's only a test for a very small part of the actual algorithm thingy. Anyway, I this script instantiates the hallway at the position of the object the script is attached to. But the trouble is, all the hallway objects have origins in the middle of them, so it spawns with the center at the gameobject. I want it to spawn at the edge of the game object, so I want the origin of the hallway to be at the edge of it. I can't just change the position it spawns at in the script, because each hallway is a different length and I am going to add a lot more. Hopefully you understand what I'm asking, basically I just want to change the point the gameobject spawns. If you need to see my script to understand what I'm asking about, here it is, but you might not need to look at it:

 using UnityEngine;
 using System.Collections;
 
 public class HallSpawner : MonoBehaviour {
     public GameObject redHall;
     public GameObject orangeHall;
     public GameObject yellowHall;
     public GameObject greenHall;
     public GameObject blueHall;
 
 
     private Vector3 spawnPos;
     private int hallChooser;
     private GameObject chosenHall;
 
     void Start(){
         spawnPos = (transform.position);
         hallChooser = Random.Range(1,6);
         if (hallChooser == 1){
             chosenHall = redHall;
         }
         else if (hallChooser == 2){
             chosenHall = orangeHall;
         }
         else if (hallChooser == 3){
             chosenHall = yellowHall;
         }
         else if (hallChooser == 4){
             chosenHall = greenHall;
         }
         else if (hallChooser == 5){
             chosenHall = blueHall;
         }
         /*else if (dungeonChooser == 6){
                     chosenDungeon = purple;
                 }*/
         else {
             Debug.Log ("You suck at coding!");
         }
         Instantiate(chosenHall,spawnPos, transform.rotation);
     }
 }