In short, while it is somewhat possible it's complicated enough that I wouldn't recommend it unless there's a really
good reason (i.e. someone is paying you per hour to do this).
WordPress does not really create multiple sites in a multisite installation. It has a single installation and uses different sets of database tables to make it look like there are multiple sites. Information about these tables and how they correlate to individual "sites" are stored in the actual (main) site's database. That's why WordPress now uses the terminology blog network
instead of "multisite" and refers to each individual "site" as a blog
What this means is that you cannot extract a single blog from the network since its data is entangled with that of the blog network (main) site which is in its turn entangled with all the other blogs. To make matters worse, WordPress holds theme and plugin configuration parameters in both the blog network site and the individual blogs. Since this data is serialized PHP data it is very complicated to merge them. For the same reason you can't directly convert from a blog network to a single site installation, dropping other blogs in the process.
What you can do is restore the entire blog network locally, e.g. using MAMP, XAMPP or WAMPserver as your local server environment, then follow these complicated instructions
. I would call this process the diametrically opposite of user-friendly but at least it clearly demonstrates why such a feature is not implemented in an automated fashion...
Alternatively, you can use WordPress export and import features to export the content of a blog and import it into a new, single site installation. The obvious problem with that approach is that everything that's not a WordPress post needs rebuilding from scratch.