The best way, imo, to go about this would be to take an Arnorian Pikeman and make a spear out of the pike by simply shortening it and assigning the proper animation in the modeldb. But I guess you'd need to also import the shield from from another unit.
Or you could just install this sub-mod. 
Edit: Oh, I guess you want it for the Eriador-era Dunedain. :/ Well, there's no simple way to do it that would avoid using Milkshape or a similar program. But believe me when I say that you don't need any modeling skills to make these sort of changes.