I don't know of a way using fill down, and I haven't tested this but I suspect you could make a field called [Selection to Add], make that a multiple choice field with the same options as the multi-select field.
Then make another formula field that is [Multi Select and Selection to Add] with the formula list(";",[Multi-Select field],[Selection to Add]). Essentially, concatenate those two fields.
Fill down on your selection to add. Then copy paste everything from the formula field, to the multi-select field.
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Mike Tamoush
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