nobody, ( I understand the fire service gets a cut of a successful prosecution)If your unable to design I would assume your unable to comission & verify the design is valid
Not true really, many electrical contractors often use a design and commision service, this can be arranged through your local wholesalers. The advantage of this is that you are only ever then responsible for the installation which is fully planned. The company who supplied the design and plans will then come and commision it.
Back to the op
You can make any area a zone, the most often used zones for a typical multi floored building would be each floor is designated a zone, and the staircase would be a seperate zone. However this is all dependant on the fire risk assessment carried out before any fire alarm is designed.
Adding anything onto any fire alarm already installed is going to be subject to various tests and compatability. Not all systems can be of mixed types for instance, and the continuity of any single circuit should not be over the maximum that the fire alarm panel manufacturers state.
All panels should be installed correctly, and have battery back up, if you turn the main power off, the panel will immediately go into fault status, after a while the panel will start to "bleep", you must have the correct codes/keys, to access the engineers facility, the user code will not give you the access level you require to add any additional devices.
You must be competant to work on fire alarm systems, it is one area where any negligence, or proved incompetancy will lead to legal action, or even death, should the system fail to raise an alarm.
There are short courses run all over the country where you can gain practical experiance in the design and installation of fire alarm systems.