I want to have a list of homebrew spells that a wizard can add to their spellbook only if they have a specific magic item (i.e. a cursed spellbook). I'm not sure how to go about this. I could have the magic item give the wizard access to another class's spell list but I'm not sure if I can do this with a homebrew subclass. I can't seem to find a way to add conditions to a spell like "spell hidden unless character is attuned to". Maybe I could not assign a class to the spells and then add the spells to the magic item? Anyone have any ideas?

What you're requesting is not possible for the system.

You can add homebrew spells to a magic item, then remove the spells from your homebrew collection (which is what is shared with other players). This way the player should only see the magic item and not the spells, but when when they add the magic item they'll get the spells added as normal.

However this only works for spells that the item allows you to immediately know and cast; there is currently no way for an item to provide access to an expanded spell list (i.e- spells you can choose to learn).