Hello,
As much as we would like to help you, the feature you are requesting is not available on the theme currently, the theme uses the excerpt of the post in the blog listing page which is dependent on the excerpt length on the customizer setting and implements the read more link accordingly.
Although the feature can be added with some advance code customization, It would be wise not to touch the code yourself if you do not have knowledge on programming,
Our suggestion to you is to hire a professional developer for your customization, so that your theme will no be effected in any way.
To hire a developer please follow the link below
http://themepalace.com/hire-a-customizer/
Best Regards.