I would rule yes. You gain access to the spell as per your Spirit, but you must prepare the spell normally. This would put it in the 5th Spell Slot and not the 3rd Spell Slot reserved for an unmodified Fireball (though you would get a vanilla Fireball as well).
I would lean no. But you could use a higher level spirit magic slot.
At 1st level, a shaman gains the spirit ability granted by her chosen spirit. She adds the spells granted by that spirit to the list of spells that she can cast using spirit magic.
The Shaman doesn't add spirit spells to the shaman spell list, only to the list of spells she can cast using spirit magic.
A shaman can spontaneously cast a limited number of spells per day beyond those she prepared ahead of time. She has one spell slot per day of each shaman spell level she can cast, not including orisons. She can choose these spells from the list of spells granted by her spirits (see the spirit class feature and the wandering spirit class feature) at the time she casts them. She can enhance these spells using any metamagic feat that she knows, using up a higher-level spell slot as required by the feat and increasing the time to cast the spell (see Spontaneous Casting and Metamagic Feats).
So a Empowered Spirit Fireball would use your 5th level Spirit Magic spontaneous slot, and take a full round action.
But since Fireball isn't on the Shaman spell list, you can't use a regular Shaman 5th level spell slot to cast it, or prepare it ahead of time.
I would lean no. But you could use a higher level spirit magic slot.
At 1st level, a shaman gains the spirit ability granted by her chosen spirit. She adds the spells granted by that spirit to the list of spells that she can cast using spirit magic.The Shaman doesn't add spirit spells to the shaman spell list, only to the list of spells she can cast using spirit magic.
A shaman can spontaneously cast a limited number of spells per day beyond those she prepared ahead of time. She has one spell slot per day of each shaman spell level she can cast, not including orisons. She can choose these spells from the list of spells granted by her spirits (see the spirit class feature and the wandering spirit class feature) at the time she casts them. She can enhance these spells using any metamagic feat that she knows, using up a higher-level spell slot as required by the feat and increasing the time to cast the spell (see Spontaneous Casting and Metamagic Feats).So a Empowered Spirit Fireball would use your 5th level Spirit Magic spontaneous slot, and take a full round action.
But since Fireball isn't on the Shaman spell list, you can't use a regular Shaman 5th level spell slot to cast it, or prepare it ahead of time.
This is correct. You can cast the Empowered Fireball out of your 5th level Spirit Magic Slot. The Shaman just can't use a normal spell slot to prepare it. Unless they have the Fluid Magic Hex.