Depends on the carrier. Most will allow you to purchase additional miles in specified increments. Depending on how many miles you need, it may be cheaper to just buy a ticket! Also, there are often many restrictions and very few seats available for reward tickets.
First, I would price the flight as if you were just paying for a ticket. Then I would check with the carrier to see if they have any available rewards seats and then check to see how many miles you would have to purchase (at what cost!)