To add a value entered in a field to select list I have a field and select list. When a value is entered in the field ,if that value is  not present in the select list then i want to append that value in the select list.
how can i acheive this?
Not exactly the workflow you're looking for but one of these two modules can help you:


Select or Other



  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.



Multiple Selects



  Rather than having a multi-select field, this modules allows you to
  have multiple select fields with the traditional FieldAPI 'Add another
  item' button.