Can I turn a pull-over into a zip-up sweatshirt? Answered

Turning it into a zip-up is straightforward, in theory: cut a straight line up the front, hem the resulting edges so they don't unravel, sew in a zipper. Folding over and hemming might narrow the body a bit; I don't know whether it would be possible to do something like wrap a separate piece of fabric over the edge to hem it instead. (As a tailor, I make a good woodworker.)

Adding a hood... First step would be to find matching fabric and a pattern for a hood. Make hood, attach. Personally, I'd suggest a hat instead, to avoid the complications.

I have seen it done! At my work we sometimes get sweatshirts from management and our union local. A coworker who is a great seamstress turn them into zip-ups.