[Phanx]

/castsequence reset=20 SpellA, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB, SpellB

... is about the closest you can get. It sounds like you're trying to make a button that casts X if it's off cooldown, or Y otherwise, and that is exactly the kind of thing Blizzard does not want an addon/macro to be able to do. They want you to pay attention to your cooldowns, pay attention to your target's health, and make your own decisions in realtime about what abilities to use.

Personally I just use a modifier key for combining a cooldown ability with another ability. It's really no more effort than having a manual reset on the castsequence, and gives me much better control over when the cooldown ability gets used.

/cast [mod:alt] SpellA; SpellB

Edit:
I'm not sure what kind of addon you're talking about with a "health conditional" but I can tell you right now that it's not automatically casting Spell A or Spell B based on the target's health without the user doing anything different when pressing the button, such as holding a modifier key or clicking with a different mouse button. You can have an addon show you some kind of alert, such as flashing a giant icon on your screen, when you should cast Spell A instead of Spell B, but the addon can't change which spell your button casts based on that information.