I want to change the author name

Here are three possible approaches:

1. A normal WP installation already has custom fields which you can add to your post. So you could use those to add the original author’s name to. The only problem is that general themes usually do not display the_meta, because they do not know what to expect. In your case you would want to replace the_author in your theme with the_meta (don’t mess with the original theme files, use a child theme!)

2. Easier, but a bit hacky: add the original authors to the user list. They don’t need to know they have an account, just provide bogus e-mail addresses and don’t let WP send their password to them. Now you can simply assign them as authors and they will have their own author page as well.

3. The most thorough approach is adding a meta box yourself (or use a plugin like WCK). This gives you complete control on how to insert the information in your child theme.