Hello.
Powering the Zumo with a 2S LiPo battery (either through the charging port or the nearby VIN and GND pins) should generally be okay, but there are a couple things to be aware of when using a higher voltage. You can find details in the discussion in this thread.
Additionally, please note that battery voltage is used to power the motors, which are intended for 6V operation. Higher voltages will result in higher speeds and more torque, but can negatively affect the lifetime of the motors and also put the motor at higher risk of catastrophic failure when exposed to excessive loads. If you want to avoid this, you might consider limiting the maximum duty cycle you so that the average applied voltage stays around 6V (e.g. around 80% for a nominal 7.4V LiPo).
Brandon