Yes — usually yes. The large side physical pads / shell tabs of the USB-C connector J6 are normally connected to GND/shield, so they are a good place to solder the converter negative if your board connects them to GND.

Before soldering, confirm with the multimeter with power disconnected:

Do not use it if it does not beep to GND.

Also: soldering to the side tabs can take more heat because they are large metal/mechanical pads. Make sure the joint is solid, and add strain relief for the battery/converter wires so they don’t rip the USB-C connector off the board.

Yes — if you mean the large mounting holes/through-holes for the USB-C connector shell tabs, those are usually tied to GND/shield and can often be used for the converter negative.

But confirm before soldering:

Do not use it if it does not beep to GND.

Also, avoid putting too much mechanical force on that hole/wire because it is part of the USB-C connector mounting. Add glue/strain relief if the wire might move.