How to upload list with multiple selections from picklist & ensure it appends the data.

We have a field titled "product interest" that essentially tracks interest as the lead interacts with our content. When uploading a list of contacts, I want to be able to mark multiple product interests and append them to the contact. I want to append the data, not replace. I believe to do that I need to utilize a form, but and struggling on how to upload multiple selections to a form. Do I need to change the field from a picklist in Eloqua so that I can pass multiple products through?