Just ran across this post. Hope it’s not too late to answer.
Your sentence, “I have a pen,” is SVO and uses the active voice. It can be rewritten using the passive voice (a pen is had by me), but it doesn’t make sense.
Sentences that can go from active to passive generally contain transitive verbs that express action, and the verb, “have” does not. You can say, “My brother lost the pen,” and “The pen was lost by my brother,” which are active and passive forms of the same statement. But the verb “to be,” intransitives, and verbs like “have” and “seem” either cannot be expressed in passive voice, or don’t sound right when they are.
Passive voice is used then the subject is not known or is not disclosed: “the cookie jar was stolen (by someone),” or when the speaker wishes to emphasize the object: “President Lincoln wascriticised by members of his cabinet.”
There is a decent discussion of passive voice at http://www.englishclub.com/grammar/verbs-voice.htm.
