What does (to be imposed upon) mean?
There is a stress on the preposition (upon). I want to know if there is any difference between when this verb comes with (upon) and when it comes with (on).
Not really- 'upon' can generally be replaced with 'on'. However, in the case you mention, I would use 'upon' if there was nothing after it:
I was imposed upon.
It was imposed on/upon us.