If Tormach doesn't recommend it, why do you want to try?
There are enough risks with stray currents/voltages in any CNC machine. Why would you want to increase that risk, if the manufacturer, who has spent considerable effort analysing and designing to minimise the risk, recommends against it?
But it is your machine and your tools so go ahead. It probably will work OK at least initially. But since you will be imposing a current onto the mill's earth circuit you will not know if you are stressing the electronics that may cause a failure in the future. Or worse still, cause an intermittent fault, the most costly of all faults.
So I would be reluctant to take the advice of someone who believes they have successfully done what you want, unless they have conducted a thorough analysis and testing of their modification.