Your environment may have custom fields that have been created for different objects that are not available from a re-importable export. Advanced reporting can be used to create a report containing these custom fields alongside other properties.
Navigate to Reports > Advanced Reporting to create a new advanced report.
When selecting categories, choose the "X Custom Fields Flat" category along with the objects you would like to report on. For an example report with devices you can use the following:
After adding categories to the report there will be a list of the custom fields to be selected in the report designer view: