I was being asked to write sentences in the passive voice, and one of the requirements was to use prepositions aside from "by" for the agent. Is that possible? Some examples would help. Thank you in advance.
You can write sentences in passive voice without explicitly referring to the subject/agent. For example, 'the ball was rolled through the tube'.
I don't think there are any other words that explicitly ascribe agency to a subject the way the word 'by' does. Instead of writing, 'a goal was scored by the red team', you could write, 'a goal was scored for the red team' or 'a goal was scored due to the unrelenting tenacity of the red team' but the change in preposition fundamentally alters its meaning and shifts the agency of the sentence.
One can engineer passive sentences where by can be replaced with via, through or using, but I can only think of examples where the agent was already identified with an instrument.
The rescue team was brought in by helicopter.
A helicopter brought in the rescue team.
You might disagree and argue that what happened was that A pilot brought in the rescue team in a helicopter, and the agent is the pilot, not the helicopter. But these sentences are correct:
The rescue team was brought in using a helicopter.
The rescue team was brought in via airlift.
Other similar examples:
Internet connection on this remote farm is provided by/via microwave.
Construction of the new prison is financed by/through property taxes.
Again, in both cases one can argue that the agent is the internet service provider not the microwave, and the city council not the taxation.
I'm not sure whether, in your question,
for the agent modifies
If the latter (not "former", as pointed out and corrected) is the case, here's an example from Oxford Advanced Learner's Dictionary:
He was too preoccupied with his own thoughts to notice anything wrong.
From Macmillan English Dictionary:
Daniel was robbed of his car, briefcase, and mobile phone.
Whatever preposition used instead of by can be argued as not related to the agent of the action.
I have been invited via an email.
I have been informed of it on phone.
In both these sentences, there is still the absence of a real agent of the actions.
I can find the only preposition that can be used to relate to the agent of an action other than by is through in a sentence like:
My ticket has been confirmed through the agency appointed by the airlines.
Here the agent of the action of confirming is the agency, not the airlines. The airlines can be said to be the agent of the action of appointing the agency.
Instead of a preposition, a prepositional phrase such as with the help of may be helpful as in:
My decayed tooth was removed with the help of a dentist.