Read post 1. It's a wiki post.
"Mechanically" certainly not. Electronically also not, as the circuit is designed the way it is, and you simply can't draw more current from this circuit as it would overheat. The components are working near their limit.
What you could theoretically do is replace the internal Mac power supply with an external PC power supply. Of course you would have to adapt the wiring yourself, as PC supplies have different connectors and pinning. The voltages are compatible, but the iMac has an SMC bus which measures the internal temperature and voltage of the supply. A PC supply does not. With that functionality would be gone, I'm not sure whether macos would continue to operate or simply shutdown if it does not detect the right voltage.
A PC supply would also not fit into the space of the iMac supply, so you would have an ugly box behind your iMac. If you don't mind, you can do that and report your findings. If you want to go down that route please open another thread and don't put such questions under "CPU upgrade" in future.