Is it possible to add options to a multi-select through deluge

Is it possible to add options to a multi-select through deluge

I am trying to update a multi-select field with a value not called out on the form, I am trying to do this through deluge, not in form. So for example a multi-select that has options {"A","B","C"} but through a script I want to be able to do something like this:

field.add("D");

Is this possible? 
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