Can a DC motor be converted into a DC generator?

Yes, absolutely. In fact, in a brushed permanent magnet motor, no conversion needs to be done. Just spin the shaft and it generates DC voltage/current.

Some can, the old kind with no electronics in them and with brushes.   All the new brushless ones, like the ones on computer case fans, they do not work in reverse, the electronics do not work in the reverse direction.

A DC motor with permanent magnets can be used as a generator, just by spinning the shaft. If it is a brush less motor, alternating current is produced, so rectifier circuitry has to be installed to convert the output to DC.