I want to create a workflow that will create a Confirm action pop up message when I am at the following page: Transactions > Order Management > Fulfill Orders > click the button "Import - CSV" (app.netsuite.com/app/accounting/transactions/salesordermanager.nl?type=importcsv) What is the base record I need to choose when creating my workflow? And if necessary, what would the record sub-type be?

I believe this is actually a Suitelet not a record. I don't believe you would be able to do anything with workflows to this page.

@AI1 Would I be able to add custom fields to this page?

If you can it would only be through SuiteScript. But even then sometimes NetSuite doesn't allow you to “break into” the code of their native screens and inject customizations. It all depends on the screen and what you want to do