class ImportExportAdsTypeResource(resources.ModelResource):
    categories = fields.Field(widget=ManyToManyWidget(Category))
sites = fields.Field(widget=ManyToManyWidget(Site))


class Meta:
    model = AdType
import_id_fields = 'name',
fields = ['name', 'active', 'position', 'categories', 'sites']
импортировать отношение m2m в django-import-export