I believe, and I could be wrong, that there's a way to do this with the caterpilar script build into BABS (Blizzs Action Battle System). Might be worth reading through the manual on that to see if it fits your needs.
Other than that, besides scripting it out yourself, the closest I can suggest is make the character an event that has an approach attribute but, thats barely close to what you want I think.
EDIT: May I ask though, why you want an actor to follow a party around a map, if that character isn't part of the party?