Need suggestions on how to extend collection_select in form

My goal: Offer the oportunity to extend the given options of a
collection_select with custom text within a form

My problem: The collection's ressource is the same the form data will
be saved to, so I cannot update the corresponding collection prior to
submitting the form as a whole.

Has anyone a smart and simple solution to that?

My goal: Offer the oportunity to extend the given options of a
collection_select with custom text within a form

My problem: The collection's ressource is the same the form data will
be saved to, so I cannot update the corresponding collection prior to
submitting the form as a whole.

Has anyone a smart and simple solution to that?