I've never known of any telephone, land or cellular, to display an extension along with a full telephone number as part oft the caller ID.
I don't believe it's possible. But if I'm wrong, your cellular carrier would know. Here's how to contact them:
OK, I reread your question and I did not answer it precisely above. You asked if John's extension is in your contacts list and a call comes in, it does not associate John's name with the incoming phone number.
Sorry I misinterpreted in my last post. I still do not believe it's possible.
Maybe a workaround would be to list both telephone numbers (one with the extension and one without) as separate numbers under the contact's name. One might be labelled "Work" and the second "Other".