Adding blog authors

Hi,

I can see quote a few threads on this issue but want to double check my thinking.

We are developing a site for a client that has lots of blog posts. These posts are created by staff in their company, but also lots of external authors which are experts in their field.

I'm unsure of how I add authors to the author dropdown? I've read that they need to be added as a user to the cms, but not sure why that's the case? The client wants to add the blog to the website and assign the author because they need to ensure it's formatted correctly, assign the categories etc.

Is there a way around this? We've imported blogs form wordpress which populates the author list and these authors are not listed as users within the admin.

Many thanks,

F.