A person is never referred to as "which".
So, it's either:
Have you ever wanted to know a seminarian to whom you could send notes and cookies or other small gifts?
or
Have you ever wanted to know a seminarian you could send notes and cookies or other small gifts to?
(here, you replace "whom" by "that", which can be omitted if it's not the subject, and you put the preposition at the end of the relative clause, because the clause cannot begin with a preposition).
PS:
What's the context?