
willuwontu |
Disclaimer: You can only use spell slots of a class to cast spells from that class's spell list (barring class abilities). You would have to create a new spell (through gm fiat and research rules) to cast a spell from another list.
Now that the disclaimer is out of the way, nothing in the rules for learning new spells (for arcane casters, whose prepared casters are the only casters whom can learn new spells outside level progression) says that those spells must be from your class's spell list, you simply have to pay according to the spell level of the new spell.

Dave Justus |

nothing in the rules for learning new spells ... says that those spells must be from your class's spell list, you simply have to pay according to the spell level of the new spell.
Other than the phrase "A wizard can only learn new spells that belong to the wizard spell lists." (Core Rule Book, where Wizard was the sole prepared arcane caster)

willuwontu |
willuwontu wrote:nothing in the rules for learning new spells ... says that those spells must be from your class's spell list, you simply have to pay according to the spell level of the new spell.Other than the phrase "A wizard can only learn new spells that belong to the wizard spell lists." (Core Rule Book, where Wizard was the sole prepared arcane caster)
Of course I overlook the header words.

Grailknight |
The witch has specific rules for adding new spells to her familiar. Like a wizard, you can learn from scrolls. You still need the hour per spell level and you the spellcraft roll, but you do not copy the spell to your book. Rather you burn the scroll and feed the ashes to your familiar in a magic ritual that all witches know. You can learn spells from witch scrolls or wizard/sorceror spells that are on both lists. Refer to d20pfsrd or archives of nethys for the official wording if you don't have the book.

blahpers |

Witches CAN use the research rules to create Witch versions of other classes' spells. Baba Yaga (Witch 20/Archmage 10) is specifically stated to have done so in order to get every single spell on the Sorcerer/Wizard list and many spells that are normally Divine.
Baba Yaga is also Baba Yaga and can do things like create artifacts and all manner of spoilery stuff. Your GM mileage may vary. But learning in-class versions of off-class spells is indeed a valid use of the spell research rules and is even called out as such somewhere.

Adjoint |

Your DM is in his rights. He has final say in what spells he'll allow to be created. Game Mastery Guide has a whole section about how to decide what spells are apropriate. The relevant section says:
Good spells expand upon the existing themes of magic, but in a novel manner. The game doesn’t really need more ways to throw damage around, but a spell that hurls adjacent enemies away from the caster is both interesting and useful. Watch for spells that break the implied limits of the game. Most arcane casters have poor healing abilities, and divine spells rarely excel at direct damage. With rare exception, spells shouldn’t duplicate existing class features or feats.

Cevah |

While most folks refer to 3.0, and 3.5, there was also 2nd ed. In 2nd ed, there was a Dragon Magazine article, and one of the rules for research was essentially: cross arcane/divine divide = +1 spell level. Simple, and effective. Making CLW a 2nd level wizard spell is poorer healing than a cleric's 1st level CLW.
/cevah