Can I pre-hash data uploaded to Facebook custom audiences?

If one wanted to upload their customer list into Facebook’s Custom Audience tool, could one pre-hash the data before loading it into the browser upload tool?

I know that the data is hashed before being uploaded to Facebook. But what if one needed to give the list to an agency that would be uploading the list for them? In order to maintain security the controller of the data would want to hash the information before handing it over to an agency.