Hi Jcspqr,
the rule I wrote is:
Spear Company (+3 points)
A regiment with a Spear Company doubles its Close Combat value when fighting charging cavalry. Unlike a Pike Company, a Spear Company does not deprive its foes of any combat bonuses. A dragoon regiment can take advantage of a Spear Company upgrade only when on foot.
This rule assume the spearmen are spread loosely among the musketeers, which seems to be the style of the Cossacks at the time. I actually wrote the rule as part of a larger resource on Eastern Europe P&S gaming which includes, among other things, an army list for Zaporozhian Cossacks. In case you want to check it out, I've attached a pdf copy to this post.