Can the firmware be modified for my particular purpose/experiment?
Yes, both the hardware and the firmware for the ArduinoTrack has been released under the GPL open source licensing, and is freely available on a GitHub repository for your specific purposes.
The ArduinoTrack_Combined is approaching its limits in terms of both flash storage, as well as RAM memory, however small modifications can easily be made to this firmware. If you are needing more capacity, then the split ArduinoTrack with the ArduinoTrack board being used as a "shield" with the Modem firmware may be better suited to your needs.
Many of the normal Arduino I/O pins are available to interface to other devices and sensors, including the I2C bus. The I2C bus runs at 3.3V and is also shared already by the BMP180 temperature/pressure sensor.