Well, clubs do fall under maces category. Although I never think of staffs as mere clubs (maybe an elaborate club).
I would say that, for the sake of simplicity, majority of traditional staffs fall under maces category, while some fall under polearms, and others under something else entirely.
Indeed, there are so many types of staffs that it's not a good idea to use any existing weapon skills, especially if it means a different skill for each type of staff. The reason is simple - there is no requirement for different types of swords, even though some are best suited for thrusting, cutting, chopping or anything in between. But practicing with only one type improves the whole category. Same with the rest.
A particular staff might have a skull attached to it, in which case it becomes a blunt weapon or an elaborate club that deals crushing damage but if that skull has horns that stick forward, it becomes a spiked mace. If the horns are really long, sharp and stick upwards, it can still be used as a mace or a club but it can also be used as a polearm or a partisan of sorts with thrusting motion and piercing damage.
This is why I think it would be best to add a new category for "Staffs" or "Staves". It will create a lot more freedom in the types of staffs and anything from a "Walking Stick" to a "Fighting Staff" to a "Scepter" can be included.
I don't know which approach this game will take with staffs. Some games have them held behind a caster's back and never actually used for melee but purely for rituals and magical enhancements. Others have you wield them like a martial arts expert in close range. Both approaches can be included too with descriptions clarifying whether they are good for one, the other or both.
