I have not tested but I would assume that you can populate into a multi select field by the format like
apples ; oranges ; pears
so delimited by a space semicolon space.
You should be able to make a native formula text field where you would take the ToText value of the existing value on that multi select field and then if it's missing one you want, then make a formula field to append on the extra choice and then have the Pipeline pickup that native text formula field to use to populate the multi select field.
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Mark Shnier (Your Quickbase Coach)
mark.shnier@gmail.com
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