How to add options to a select list of a content type?
It sounds like using a taxonomy would be more appropriate here. Taxonomies allow lists and terms can be automatically created from user input, in this example something like the Select (or other) module would be of use. Details from project page:


  Provides a new Forms API element which is a select/radios/checkboxes element that has an 'other' option. When 'other' is selected a textfield appears for the user to provide a custom value.