It is far, far better to create your own scripts and dialogue stuff for new followers than to edit/amend the existing vanilla stuff/quests.
Especially when you want a type of follower that doesn't already exist.
If you can follow them, there's instructions here:
http://www.gamesas.com/topic/1349193-wip-an-unexpected-companion/
Look at POST No16 and the amendments to that proceedure in POST No19 (with thanks to jediborg and DreamKing, as ever

)