How a d.c generator can be converted to an a.c generator?

Normally, it can't due to its design (the collector is the culprit).
AC current can be rectified to DC, so if you combine an AC generator with a rectifier, you could call this a DC generator (not entirely correct though). And this so called "DC generator" could be "converted" to an AC generator, simply by removing the rectifier.
Likewise, to a DC generator you could connect an undulator, which converts DC to AC, and call this combination (incorrectly, again) an AC generator.
An DC generator is called a "dynamo", an AC generator is called an "alternator". The "dynamo" on your bike actually is an alternator, so it is called a dynamo incorrectly (a historical error).