Please refer to this documentation on creating custom fields using the API.
Adding custom fields for only one list is not possible. When a custom field is added it applies to all lists. The same rule applies for all content modules in Sitefinity because custom fields are tied to a object type rather than a single row of data. Consider a scenario how each row (in the same table) would have different columns, which is not possible by database rules so custom fields per row (or list in this case) is not possible as well.
If I have created some custom fields in List Itmes then can I access them using fluent API?