I'm trying to export each response as a PDF with additional questions for a team of reviewers to answer the added questions?
I copied my form with the answers, added the questions I wanted to the copied form, exported each respondent as a separate PDF. Then I combined these forms into a single PDF - but the questions I added are not fillable. Is this doable?
No. You can edit the PDF(s) you exported using Acrobat Pro, but the data on the pdf page cannot be converted back into a field. Further, sending out your form copy to the respondents (using HTML) will show a new form with no answers.
What you would want to do in the future is try the Show/Hide functionality. With this feature you would design the form so that based on answers the user has provided, you could show follow on questions. This is something that needs to be done at form design time.