If an emf is produced in a dc motor, would it be possible to use the motor somehow as a generator or source, taking power out of it rather than putting power into it? How might this be done?
Yes an electric motor will also work as a generator. That is, you can connect it to a load and when you rotate the motor it will push a current though the load. There is a thorough explanation of this in Farcher's answer to Eddy current induce emf.
However designing a generator for maximum efficiency is a complicated business, and your motor will not be as efficient a generator as a generator that has been specifically designed for the purpose. The differences between motors and generators are explored in How does the electric motor and generator vary?