Looking at mine I`d have to say no, not really. In the middle (on the back of the board) is where the 2 pins go back and stick into the holes where the bulb used to go. In theory you could perhaps drill an extra couple of holes of the correct spacing and solder an LED there but it would be seriously overdriven and, being right in the middle of a load of others, probably would overheat since it would have no cooling. There is little room to add a resistor.
Of course Tektite get away with overdriving 19 LEDs in a similar sized array in their Expedition 1900 light (I think- I still can`t afford one of those so I can see myself) so it might be OK. I`m not sure but in the case of the DB18 I think an extra LED would need an appropriate resistor to limit it.
When you say there is a hole in the beam- what do you mean? What colour did you get- my white one gives a very smooth (ever so slightly violet tinted) beam with no rings or holes. Other LED colours might have rings in their individual beams and in that case you really can`t doo much to improve it by adding an extra LED.
