Let’s assume that your prefix is wp_, your blog ID of your main site is 1, and the blog ID of your sub-site is 2.
In the wp_usermeta table all rows with a meta_key starting with wp_ (and not any number after it) are applied to the main site. To make them apply to the subsite, they need to start with wp_2_ instead. This will give the users roles in the site with blog ID 2.
You may also need to take the row from the wp_options table of the original single site, that has an option_name equal to wp_user_roles and put it in the table wp_2_options of the multisite, but also change the option name to wp_2_user_roles. This will move the available roles to the site with blog ID 2.