There are companies that make USB to 12 volt adapters. Here's one example I got from a quick search. https://www.amazon.com/Cigarette-Lighter-Socket-Female-Converter/dp/B07J2JPWFD/ref=asc_df_B07J2JPWFD/
This provides 10 watts maximum, and that's about all you can expect from a USB port. There's ample USB power bricks that are rated to supply 12 watts but I suspect you are trying to get this power from a USB port on a computer. With the trend for people using USB to power devices I've seen USB ports getting "amped up", some supplying 12 watts like the power bricks so many people use to charge up their cell phones and tablet computers. As an example Apple has been doing this since they introduced their USB 2.0 iPods. Most computer makers though allow at most 7.5 watts from their USB ports before some over current protection kicks in, with lower maximum power ratings also possible.
Trying to connect multiple USB ports together to get more voltage or current is playing with fire. This would be especially so for putting USB ports in series to get more voltage, the ports almost certainly share a ground and connecting the ground of one USB port to the +5V of another USB port is just going to short circuit the power supply. The most likely result is the computer shutting down to protect itself. Another possible result is smoke coming out of something.
There are Y-cables out there that will take power from two USB ports to get around this power limit on USB ports but this violates the USB spec and so can have unpredictable results. Unpredictable as in you could see smoke come out of something. It's certainly possible to get power safely from multiple USB ports but this will not be cheap or easy.
It's not "obvious" to me that a 12 volt motor cannot be run from a 5 volt supply. Running motors at voltages lower than their rated voltage is practically routine. How your motor would respond depends on how it's built. Running it on 5 volts could let some smoke out. Simply running the motor on 5 volts may be an option.
Check the power consumption of the motor. If it's under 10 watts then you might be able to buy or build a booster circuit that supplies 12 volts from a single USB port. Check the USB port to see if it is able to supply enough power for your motor. If you need to boost the voltage then the USB port needs to supply enough power to make the motor happy and make up for losses in the booster circuit.
Rather than trying to combine the power from multiple USB ports see if you can't split the load. It's been a while since I had to work on motors at university but I can recall that there can be more than one magnetic coil in the motor. Perhaps you could power the separate coils from separate USB ports.
I would highly discourage combining the power from multiple USB ports, again that's playing with fire.