One thing to keep in mind is that exterior doors open inward. So, if that's not a problem for you, there's the sizing to think about. Exterior doors typically run 36" wide, whereas interior doors are not as wide (30"). But maybe you already have a door in mind that fits the space? If so, there's no need to deal with insulation and weather stripping. You can use the existing frame, but it's definitely a challenge to make a door hang properly. Just exercise patience when doing this, and you should be fine.
In theory, it could be trimmed. But here are some potential problems:
- Like you say, the composition. A lot of exterior doors are metal-clad wood, which would leave you a ragged edge on the cut side.
- Even the wood doors are not solid slabs of wood. As the image shows, they are composed of various pieces that may look very strange on the cut side (and may even lose their stability).
- Aesthetic considerations: if the door has a paneled appearance, the panels will no longer be symmetrical if you trim off one end. You would have to trim off both ends.
- Precise cutting needed. You can't make a straight cut with a hand saw. Even with a table saw, it will be difficult to make a perfect cut. You would have to have a professional-size table saw that could accomodate the entire door.