can a dc generator be converted in to an alternator?
Yes. Just change the split ring commutator to a slip ring commutator.
Actually the EMF induced in rotor it is an alternative EMF. The commutator acts like a rectifier and the brushes collect a rectified d.c. current.
As Aceix said, if instead of commutator you'll fit sliding rings you could get alternative-single phase current. However, you have to check how to connect the coils in series and parallel in order to get the required voltage and current.
In order to get three-phase system -except three or four rings-you will need to modify the windings also.
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