The Smart Citizen Kit is comprised of two stacked PCBs, a bottom one dedicated to data processing and communications, and a top one dedicated to the sensors.
The connector between the data board and the sensors board has a standard connector in order to provide power, analog and digital communications (ADC, GPIO, I2C, VCC). Currently a single sensor board is available: the Ambient Board (Check the Hardware specifications on the section above on the Ambient board sensors specifications).
Anyone can take the PCB design files (The Smart Citizen Ambient Board design files https://github.com/fablabbcn/Smart-Citizen-Kit/tree/master/hardware/Kickstarter/v1.1b/Ambient_Board_v1.1b) in Eagle and develop a compatible board. This implies also writing the firmware library to support the sensors taking Smart Citizen Ambient.h library as a reference (The Ambient.h library is part of the Smart Citizen firmware an is dedicated to sensor management https://github.com/fablabbcn/Smart-Citizen-Kit/blob/master/sck_beta_v0_9/SCKAmbient.h). This is a good option in the case that a user wants a completely different set of sensors, while keeping a small form factor, and plans to make more than a few units.