No.

Cantrips are never prepared, they are only learned, and from then on they are known and available to cast. They explicitly cannot be copied into a spell book because cantrips do not fill nor expend a spell slot. From the spellbook inset on PHB p. 114 :

When you find a wizard spell of 1st level or higher, you can add it to your spellbook if is of a level for which you have spell slots and if you can spare the time to decipher and copy it

So no, you can't add a cantrip to your spellbook. As far as learning more cantrips than you are allowed in some way, I don't think it's viable to learn more than your class level allows without a multiclass or a feat.

The arcane discovery alchemical affinity implies an alchemist/wizard can't normally copy from his formula book to his spellbook

Part of the benefit of the arcane discovery alchemical affinity says, "Additionally, you may copy spells from an alchemist's formula book into your spellbook just as you could with another wizard's spellbook." So Magical Marketplace's author, at least, believed it normally impossible for a wizard to copy into his spellbook a formula from an alchemist's formula book.

(Further, the alchemist's supernatural ability alchemy says, "A wizard, however, cannot learn spells from a formula book," and while Pathfinder isn't as clear as it should be when tossing around a very pregnant word like learn, Adding Spells to a Wizard's Spellbook nonetheless says, "If the [Spellcraft skill] check fails, the wizard cannot understand or copy the spell" (emphasis mine). The text continues immediately after, saying, "He cannot attempt to learn or copy that spell again until one week has passed" (emphasis mine). Parallelism makes understand and learn synonyms here, so a wizard, even after waiting a week, still won't be able to make that Spellcraft check to copy a formula because a wizard can't learn spells from a formula book in the first place. Note that I don't like this ruling as much as I like the anecdotal evidence from Marketplace because whenever the GM must make a ruling based on a rule's syntax there's usually more than one possible reading, making disagreements and hard feelings a possibility, so make of this what you will.)

A house rule could allow copying formulas into spellbooks anyway, despite the existence of alchemical affinity

A GM that makes a house rule that allows the PC to add his handful of appropriate alchemist's formulas to his wizard's spellbook (and still requires time and money to transcribe them) needn't worry that the PC is attempting some kind of crazy power-grab. Under such a house rule, a wizard who, late in his career, decides to take levels in alchemist trades versatility for power… and makes a completely uneven trade.