There are indeed no tespoints but it can be done like David said.
I would refrain using a PSU or even standard batteries in series though.
Standard batteries means using a holder and some cheap crappy ones are prone to contact micro-interruptions when you move of bump them. You also have the risk of one cell popping out of the holder while manipulating it.
If that happens when the original cell is unsoldered, you are toasted...
Same thing with an external PSU with banana cables, mini-grabberss, etc.
What I do is use a Lithium cell with leads, like the ones meant to be soldered directly to a PCB, and I solder a 5K resistor on one end in series with an isolated wire and a second wire on the other end.
Then I solder both wires to the scope board on a place where tracks lead to the battery.
But you must connect after the original battery 10K series resistor on the PCB, not directly to the original battery.
That way, you are protected for any short you make when unsoldering the original and you won't load any of the two batteries which will be at different voltages.
Final precaution, don't ever do that with the board connected to anything, particularly earth.
Your iron may be earthed so if your board is earthed because still connected to scope and scope switched off but still connected to grid, when you will touch the positive of the battery with the tip, you will short it through the earth loop.