The type of battery in the battery jump-starters is very good for an electric bicycle, but you will need 3 of the 12V's to get 36 volts, and 4 to get 48V (wired in series, not parallel).
The starter pack should have come with a charger. If it's one of the "affordable" brands, it may not have a State-Of-Charge (SOC) sensor, so, if you leave it on for a long time, it will continue to charge the battery after its "full". That would be bad.
A more sophisticated charger (which it "might" have) will give it more charging amps when its very low, a trickle when its near full, and then stop when its done.
To get a generator to make electricity, you need to spin magnets next to a stationary set of copper wire coils. If the magnets are "permanent" magnets, such a generator is called a Permanent Magnet Alternator (PMA).
Some enthusiasts have taken a Permanent Magnet Motor (PMM), such as a "servo motor", and its easily converted to a PMA.
The type of motor in your drill probably has will use copper wire coils aound steel cores, and by passing a flow of electricity through them, the spinninmg coils become "electro magnets".
If this is the case, you could make it an "induction motor conversion" where you add capacitors to provide "start-up" electricity to the coils in the spinning armature, and after starting, a small amount of electricity would be drawn from the field coils to keep it going (through brushes).
If you took the armature out, and replaced the spinning copper wire coils with permanent magnets, the search term is "Zubbly conversion".
Even if you went through all this effort to make your drill a generator, I am certain you will be very disappointed with its output (plus the bearings were not designed for continuous duty).
Do you want to charge a 12V battery for some purpose? If you specify the desired end result, you may be suprised by the useful suggestions....
