It's definitely possible with a masters in SE. There are many types of quant (see the markjoshi.com link above) - it will be easiest for you to become a quant developer with your background in software engineering. This will be much more focused on the software development side of things rather than the math/modelling side of things, but depending on the firm will give you large exposure to how the modelling side works. They are very competitive positions, but if you are a strong programmer that can prove you have knowledge of financial markets, have relevant side projects, and can understand and communicate the math behind what you build, you will have a leg up on the competition.