Powering Adafruit motor shield
You should not power the motor shield (and thus the motor itself) through the Arduino. The motor draws a high current (depending on the exact product - there are many versions of the nema-17 - I found current in the range of 1 - 2A) and the voltage regulator (or the diode in the path) are not rated for this much current (for the Uno the absolute maximum is 200mA, including the Uno itself). You will fry these components, if you try to draw more than the rated current.

But you can use the same power supply and provide the power to Arduino and motor shield in parallel. This way the current for the motor does not have to pass through the Arduino.