Spells require combo points in the AttributesEx column.
ee the
Wiki for this.
In my mangos core (which is quite old...) the flags for combopoints are:
SPELL_ATTR_EX_REQ_TARGET_COMBO_POINTS = 0x00100000
and
SPELL_ATTR_EX_REQ_COMBO_POINTS = 0x00400000
You have to set them for your Spell.
In a comment in the server source it's indicated that warriors use combopoints, while not shown ingame, to indicate whether or not to be able to cast overpower. I'm not sure if that is a hackish piece of code (but since mangos doesn't usually use hackish code I doubt it) or not.
So I think you can add the combopoint flags to a mana spell and then may need some server side modifications and it should work. For client side display of the points I think you'll have to make some ui changes.