The short answer is YES – but you don’t have to pass and register a Special By-Law in order to resolve the present issue involving the impending sale of a Lot. As an “easy-out”, your Plan’s Executive Committee could issue a retrospective consent to the Owner of that Lot, covering the air conditioner and the requirement for all on-going repairs / maintenance / replacements to be the absolute responsibility of the Lot Owner from time-to-time (i.e. current and all future Owners).
Then … you need to have some generic Special By-Laws expertly drafted, agreed to at a General Meeting (<25% non-agreement), and Registered to retrospectively and in future cover the installation of air-conditioners, the placing of items (solar panels / solar hotwater) on Common Property, and Additions to Common Property (the sun room, and in this case probably covering that Owner’s Exclusive Use of that area).
You need to resolve this, because if there's ever a discussion about who's responsible for those items in the circumstances that you describe, there's precedent for all of them to be deemed “Common Property”, and in which case your Owners Corporation will be entirely responsible for all maintenance / repairs / replacements; forever!