Answer: Yes. The easiest way is to make an “adapter” that fits on the regular 9 V battery connector of the detector and connect the external battery through the adapter. The “adapter” can be made of a standard 9 V battery connector with leads:
The external battery is connected to the leads of this connector with the positive terminal to the black lead and the negative terminal to the red lead. Please note that this is opposite to the common standard and this is because the battery connector is used in the opposite direction in this case (to supply current rather than to receive). Before connecting an external battery to the detector using the above adapter, you should carefully verify that the polarity at the battery connector is the same as that of a 9 V battery, e.g. using a multimeter. Reversing the polarity may cause permanent damage to the detector.