Is it possible to make intransitive verbs into a passive form?
Most of the representatives said they plan to take the subway train to the convention building rather than ride a cab paid for by the sponsors.
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Indeed it is possible to use a passive voice for intransitives, as long as you use the proper prepositions:
The teacher talked to the boy: The boy was talked to by the teacher.
It isn't as sharp as the active voice, and could be confusing in longer sentences however it is perfectly possible as long as the "to" is used, or the appropriate preposition.
Most of the representatives said they plan to take the subway train to the convention building rather than ride a cab [paid for by the sponsors].
The bracketed clause is passive by virtue of the fact that the bracketed element is a past-participial clause modifying “cab”.
The key point is that past-participials as modifiers are ‘bare’ passives, i.e. they lack the “be” and “get” markers, but they are still passive as evident from the admissibility of a by phrase, as seen in your example.