CSV files are preferred when working with ForeFlight Logbook for importation. You may want to ask ZuluLog about the best method for exporting your logbook, but a good starting point is to follow the steps listed on their website here:
You can upload your ZuluLog CSV file directly at plan.foreflight.com. Keep in mind that the fidelity of information can vary based on the setup of your ZuluLog logbook, and direct import may require adding information - such as aircraft details - in ForeFlight Logbook. We recommend using the template offered on the import page for maximum detail.