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of course you can! Unless the motor requires 8 volts or more to start, it should work. Providing a 7.4 volt will obviously not turn as fast when you provide 12 volts but it should work. If you need a 12 volt constant output for your motor, you could use a step-up voltage regulator. These sustain a constant voltage output from varying input voltages.
It will depend on the exact motor but most small brushed motors will be fine provided the load, and so the current, is not too high. It will just run a bit faster than normal.
Steve