I want to fit a Hub motor ($15kW$ BLDC motor) to my car to convert it into a hybrid vehicle. Would it be possible for me to use the same motor (as a generator) to charge my battery pack, while the vehicle runs on it's normal Internal Combustion engine?
There is no theoretical reason for this not to work, but there are many practical considerations, such as the fact that BLDC's typically produce 3-phase power, which would require extra components to convert for use as a charging source, and there may be issues with back-driving any internal gearbox that may be present. E-bikes regularly include regenerative braking functionality, so you could look into this. Without more details on your exact setup, we can't be of any further help.