How can I add additional Value in a Choice List (Fixed) Multiple Select Choice List? using function

Summary:

Ok so there is an existing field that has multiple values from the Choice List, and I am trying to setAttribute but do not want to overwrite the current values existing in the field I am having trouble on how to add it without affecting the current values. as it does not work like an append on a normal code

Content (required):

I am trying to setAttribute but do not want to overwrite the current values existing in the field I am having trouble on how to add it without affecting the current values.

Version (include the version you are using, if applicable):