It’s possible but not straightforward, the only difficulty is there is a pin on the cooling bridge that is used by the hub PCB to determine if there is a Raspberry Pi in the device - it won’t turn on unless this is detected. You’ll have to:
- Use a female to male HDMI cable to connect the hub to your external pi
- Dock the cooling bridge into the hub inside the pi-top and then use a jumper wire to join the pins I’ve connected with a red line (this is a view from the top of the cooling bridge)
I think this should get it working, but let me know what happens and we can debug if necessary