Rae from Shopify here. Thanks for reaching out with this question.
At this time, the author of each blog post can only be attributed to the store owner, or to an employee with a staff account on your site. We have more information on this in this help document under step 5.
With this in mind, I recommend creating staff accounts for the authors of your blog posts, but restricting their account permissions so that they can only view certain parts of your store's admin. This will help protect your store's information, and ensure that any details you don't want viewed by staff are kept private. Once you've created staff accounts for these individuals, you will still be able to add in the blog posts from your end, but now their names will appear in the 'Author' drop-down selection, so you can add the correct person as the author of each post.
I understand this workaround may not be completely ideal for how you handle staff accounts on your site, so I'm going to share this information with our developers. Our teams review comments and feedback like this as a means to improve the features on our platform, so these details are really helpful for us to receive.
Please let me know if you have any questions or concerns about this set up, and I'll be glad to continue helping out!