If you use a ready-made “power bank” with built-in charge control and step-up voltage regulator, then that depends on the design of your power bank. Hook the bank to the load, and hook up the charging cable to the bank, and see what happens.
If you’re using a plain battery (3S or 4S LiPo, or 4S or 5S LiFePo, typically) then you can parallel a DC voltage with the battery, assuming the DC voltage is reasonable for the battery. This will create a “float” on the battery, which for LiPo batteries is not great; you should not float LiPo batteries at their full charging voltage, because this will significantly degrade the life time of the battery. I’ve successfully floated LiPo batteries at lower voltages, such as 3.9-4.0 volts per cell, but I haven’t measured the amount of lifetime degradation; theory says there should be some but not as bad as for the top voltage. The other benefit of “floating” a battery like this is that when there are temporary large power draws (such as if you have motors involved,) the battery serves as a buffer, and dampens back EMF and voltage swings. I’ve blown out electronics before by feeding a switch-mode power supply straight into a motor without buffering, and the power supply swinging the voltage way too high when starting/stopping the motor.
If you’re using a power bank with voltage converter, OR if you don’t want to float the battery, then you need to create a cable that connects each of the power and battery through a diode; typically a forward-biased low-loss Schottky type to avoid losing too much power. At 19V, you may see up to 2A (3.5A if you have additional external devices plugged in) so make sure the diode is rated at twice the expected current to avoid overheating it. If the battery voltage is higher than the power supply voltage, and the power supply isn’t designed to be safe against back-feed, you’d also want a diode on that input, so the safest thing is a cable with two inputs, both of which are diode protected, merging to one output going to the barrel connector on the Jetson.