I am curious if it is possible to change the 3.5mm jack output on the raspberry pi to use it to charge a jawbone up fitness tracker?
No; The raspberrypi headphone jack (0.3mW, 1V) will not output enough watts for the jawbone (2W?, 5v? likely set to max out USB).
You can skip the raspberrypi and just use the USB power supply directly. If you have lost your headphone to USB adapter you can (assuming the adapter is only for shape) desolder a headphone jack from a raspberrypi but then your pi would be limited to HDMI audio.