The usage is called "passthrough." The good news is that the PowerPack 10K does support passthrough. But, there are a few caveats to keep in mind:
1) You need an AC power adapter (power brick) with an adequate power rating. A power rating of 10 to 18 watts would be good. But, it doesn't necessarily mean using a wall charger with a lower power rating won't give you passthrough. The battery pack takes precedence when there's not enough power to go around. But, the battery pack will eventually get fully recharged, by which time, the surplus power will get routed to the device connected to the USB-A port.
2) In the passthrough mode, the output voltage from the USB-A port is limited to plain 5 volts only. There's no fancy fast charging or anything like that.
3) Use high-quality cables with low electrical resistance. The battery pack does not compensate for any voltage drop in the passthrough mode.
4) The PowerPack does not work as an Uninterrupted Power Supply (UPS). During the passthrough setup, if the AC power is cut, there will be a momentary loss of power on the output port of the PowerPack. That split-second loss of power is enough to cause a device without a built-in battery to reset.