Add user to user list field from excel import of lookup field with pipeline

I am currently trying to add a user to a userlist field named "assigned to" on file import that uses the value in a lookup field that is based on another table of credit card numbers and the credit card holder name. This field changes users consistently so adding it to the credit card information table will not work. 

Dynamic rules do not apply on import so what steps in a pipeline will do this for me?