Can ANY DC brushless motor be used as a generator?
Brushless DC motors are usually motors that have permanent magnet rotors. It would be extremely unusual to find any other type of motor described as a brushless DC motor. All such motors can be used as generators, but some designs are easier to use as generators than others. A major example of a difficult motor is a BLDC fan motor found in a computer. Those have electronic circuitry built into them that must be removed or disconnected in order to use the motor as a generator. You might find some other design described as a BLDC motor that would be difficult to use as a generator, but most of them only require the shaft to be turned to produce AC at the terminals and a rectifier added if you want DC.