Adding a new fireplace to upstairs bedroom. Want to tap into existing unused vent
I will start by saying, if you're willing to spend the time and/or money, almost anything is possible.  My gut feeling is that a wood burning fireplace would generally be out of the question without permits, engineer drawings and significant modifications.

A gas burning fireplace is definitely doable along an exterior wall using direct vent models, bypassing the chimney all together.  Some models can also use the chimney and I would think that as long as your current chimney can provide the necessary clearances of the exhaust vent, it could be used.