MySQL is a requirement for running wordpress. Asking if you can run it on another SQL DB is like asking if you can run it on the new hack language which is similar to PHP. It might work or not, but basically you are totally on your own, and for sure it is not something that you do as an afterthought after the project is almost done, this is something you should start the development with.
if you still wish to investigate the possibility, you should start with replacing the DB access driver, the WPDB
class, with a class that implements the same interface but uses the mssql related php api instead of the mysql one. Even after that you will have to test well as much functionality as possible as the SQL supported by the various DB is almost, but not exactly, the same.