Well, if you don't need to extend with any other functionality, I would make the "Owner Quest" of the package the VANILLA QUEST, that way the AI Package will be able to access that specific NPC alias.
You should then be able to assign the package to your new NPC with no problems (I hope).
Tried this,but while the package will detect the aliases, the template remains greyed out - even if it too is owned by either quest. All I need is a target to follow, so both aliases run at the same time.
Otherwise, if you need your new quest to be the Owner Quest, then you'll need to create a new alias.
You can get a referencealias to basically copy another quest's alias:
Select "External Alias Reference", select the vanilla quest, and the name of the alias in the vanilla quest. You now have a quest alias that directly mirrors a vanilla quest alias.
Also make sure "Allow Reserved" is checked.
Tried this too, and again the Template has the same problem, no matter which quest owns it. It stays greyed out. I would have thought the template gets overwritten but its seems this is not always the case. Could it be a bug? The "Allow reserved" is also greyed out when quest enabled is checked.
Finally, note that aliases are ONLY filled when the quest STARTS. They are not updated afterwards except through direct script commands.
So when you are testing, make sure you are starting a new game!
I can't really ask users of this mod to restart their game. They would quickly hit the unsubscribe button. I already lost some when I accidently uploaded a BSA with scripts missing.

Any idea what the script command is? I came across this when the player horse dies and respawns
MySelf = GetActorReference() [Actor property associated with the NPC reference]
Alias_NPCRef.ForceRefTo(MySelf) [ReferenceAlias property associated with the alias to be followed in the vanilla quest]
so I could add an aditional line,
Alias_FollowTargetNPCRef.ForceRefTo(MySelf) [ RefAlias property associated with the alias in my quest]
Although I can't see how that would fix the problem with the AI Template, where the it always remains greyed out regardless of which quest owns it. I think this is the main problem.
The Two NPCs involved is the Whiterun Player horse (stables quest) and another horse in my quest. All I'm trying to do is get my horse to follow the players horse, when neither is being ridden. Sounds elementry on first glance but its not as straight forward as following the player.