BigCommerce allows users to create custom fields. Custom fields can be configured in the Custom Fields panel.
Please see the image bellow:
a) Use the column on the left Pimcore Custom field to set you Pimcore product fields.
b) Use the column on the right BigCommerce Custom field to set your BigCommerce custom field. You will need to make sure the name of the BigCommerce custom field is typed in correctly, otherwise the sync will fail.