Yes, you can, as long as your life jacket is fitted with a SECUMATIC 3001 S or 4001 S type of inflator. We carry a manual override for both of these inflation devices in our range.
You will find more info under products -> accessories.
If you have one of the older inflation devices in your life jacket, then it is not possible to switch off the automatic function. The older inflation devices do not have indicators to show whether the life jacket is operational or not, so generally you would not know the exact status of such an inflation device.