Jenkins Job Builder Doubts using !join operator

Question

I have few questions about how to use the Jenkins Job Builder, especially, with the !join operator.

Let's say, I have the following YAML sample:

    - extended-choice:
    name: OPTIONS_CHECKBOX
    type: checkbox
    value: !join:
        - ','
        -
            - OptionA
            - OptionB
            - OptionC

I would like to have the OptionA OptionB and OptionC come from a different file. I tried creating a list in another YAML file with the default parameter and referencing that here using a job template. But, I was unable to do so. I would like to keep the list separate because the list has around 50 parameters and I would like to track it separately. Is this possible?

Also, I have one more other question. Is it possible to only update a section in a Jenkins jobs. For eg. I would like to only update the "parameters" section in a Jenkins job and not the entire job. How do I do so?