Does anyone know how I can append data from a Deal custom field to a Person and Org company fields using a workflow or automation process?

I have the same custom field across Deal, Person and Org records and when a new Deal is created and the custom field updated, I want the value(s) to append to the Person and Org record custom fields, but have been advised by the support team this is not possible.  Wondered if anyone knew of another tool that could help me achieve this?  The field is a multi option field.