WHat do you mean by "options available to fields". Do you mean you want to add extra Choices to a Choice field used in the list? If so, create a feature with a feature receiver and access the SPList, the the available fields in the list, get the field you want to update, Cast it to a SPFieldChoice, add the new values to the Choices collection and call the field's Update method, then the SPList's Update method.
If the field in question is a field available as a site column in the entire site, get the RootWeb's Fields collection, find your field and perform the same actions to add choice values. This time, call the Field's Update mathod with true as parameter, so the changes will get pushed down to any list using the field. Then call the RootWeb's Update method.