Q: Would the PowerPack 10000 fast charge the Galaxy S9, S9+? Would this power the Galaxy Gear S3/Frontier watch's wireless charger?
A: Yes, it will fast charge your Galaxy S9 fine with the USB charging cable that came with your Galaxy.
It should also power your wireless charger via Micro-USB to USB cable. Just plug the micro-USB end into your wireless charger and the USB end into the PowerPack. However, keep in mind that the wireless charger may not draw as much current as your S9. If you plug the PowerPack into the charger and it fails to work after a minute or two, you may need to try the low power charging mode or light-load charging.
This battery pack also has a specially designed lower power charging mode that can charge low-power devices where other battery packs fail. Lower power devices like smartwatches, Bluetooth headphones, and fitness trackers may just draw a little current during charging that most battery packs on the market would simply decide there's no charging and hence proceed to shut down. By contrast, this battery pack can charge in low power charging mode (entered by double-pressing the button on the battery pack). To exit the low power charging mode, either wait for the 2-hour timeout or single-press the button.