Question

How to convert the dc generator into an alternator?

Answer 1

Simple technique:

Take the DC machine to a specialist dealer
Trade it in
Use the money to buy an alternator.

Complicated technique

convert the commutator to a slip ring less than half the width
Add a second-hand slip ring next to it
Butcher the brush arrangement so one runs on one spring, the other on the other
Trace the ends of the armature coils and split them among the two slip rings
Add a DC exciter to feed the rotor via the slip rings
Take the output from the former field windings

You end up with a rather poor alternator, whose maximum output current, from the stator windings, is much lower than the original design.