You need to edit in the Descr_moddel_battle file in order to do that. There might be some issues though.
One, and guessing that Pergamon share the animation with a lot of other hoplites, any change will affect the units that share the same DMB entry.
Other, the skeletons involved are probably, fs_spearmen and fs_o_f spearmen or fs1_hoplite. What might happen is that the spear positions might be reversed and you will end with the unit stabbing with the buttspike. That might happen or not.
I sugest finding a good tutorial if you want to do that and back up the DMB and EDU before starting.