How do you make a prefab into a tree? Or is it possible?

I downloaded free palm trees from the asset store and when I use the terrain tree paint tool, it doesn't work with those trees. I don't understand why this is happening but I'm sure it might be that fact that it just doesn't have the right parts of a tree to be painted. But it already has a prefab( I could mass place the prefabs with a prefab brush I have, but that would lag the game).

I only have 2 choices: I place the trees with the prefab brush and take on the lag OR I need to figure out how to make that tree usable in unity terrain brush tool

So the question is: How do I accomplish this? How can I make the tree prefab into a valid tree so that I can use it in unity terrain tools. And it would also help if someone could tell me this:

If I have to use the prefab brush, how can I prevent my game from lagging? I know it's because I there is so many prefabs but is there a way to load them and unload them when I'm in the tree's view? If this doesn't make sense look at this code:

using UnityEngine; using System.Collections;

public class renderGrass : MonoBehaviour {

 // Use this for initialization

 //public float drawDistance = 0.5f;
 void Start () 
 {
     Terrain.activeTerrain.collectDetailPatches = false; //draw distance is optional
     //Terrain.activeTerrain.detailObjectDensity = drawDistance;

 }
 
 // Update is called once per frame
 void Update () {
 
 }

}

Let's say I painted a bunch of grass in my terrain. It will lag a lot right? This code makes it so the grass is not loaded or seen (rendered might be the word) until I approach it. That's its purpose. I want to do the same concept for placing trees with the prefab brush if I have to.