If you are doing Brownfield development (which you are), then you can go one of two ways:
- Create the Entities / Controllers / Views manually
- Use the Scaffolding generator to create the above for you
Depending on how complex your existing database might be, you would be able to choose one over the other. If your existing database's schema is complex, has field names that are not what you would want in your entity you may want to go with option 1 as well as using ClassMaps instead of the automapping from Fluent NHibernate.
Also, depending on how many tables you have as well as how large they are (number of fields), using the Scaffolding Generator might still not be a bad option. To give you an idea, I've used to in a brownfield situation several times with rather large medical databases. It went much faster than me recreating those manually.
In the end, you really need to decide which way would be faster for you as well as which one would be more relevant. We have a pretty strong community with S# so getting some support is not an issue.