The main issue with getting a field value from the main site is that the field in question must be defined on every site in order to use the value from the main site, there are several ways that this can be accomplished.
If the field exists on both the site being shown and on the main site then you simply need to use switch_to_blog() before getting the value.
If the fields are not defined on both sites then you need to use switch_to_blog() and then use get_post_meta() or get_option() or some other function in WP to get the values of the fields.
