@Popatop15 I'm sorry, but it's near to impossible. Take a good look at those examples https://github.com/NodeBB/NodeBB/tree/master/src/database
The differences between MySQL and any noSQL databases are huge.
NodeBB architecture was designed with Redis in mind. Redis it's not even a database at all, it's an advanced key-value store (completely noSQL). So, it's relatively easy to make a driver for a noSQL database, like MongoDB. But it would be way too hard to implement that kind of a driver for a relational database, like MySQL.
Just to be clear - even though, strictly speaking, it is possible to make the driver, NodeBB won't be working good with it anyway.
Personally I'd suggest you to spend some time to learn how to work with MongoDB, if you still unfamiliar with it. MongoDB is very popular nowadays and working with it is much easier than with anySQL database. If you're an expirienced dev, you can become a ninja in just one week.
Then, you'll be able to make an effective bridge between Mongo and MySQL parts of your applications.
p.s. Just in case, I want to mention that everything is possible.