.CSV upload - how to modify uploaded data while it's uploading?

I have an issue with CSV uploading that I can’t seem to figure out.

My ultimate goal: user can upload a csv file with their info into the database and it will populate their account with the relevant data.

The data type being uploaded is called Contractor

When I upload the csv, it looks like this:

I believe that’s in part due to the fact that for Contractor type, the field Account Manager is set to User.

Does anyone have a way to manipulate the data so that when the CSV uploaded, the application knows to assign the Account Manager as the Current User?

Thanks!