In Marketplace Multi-Vendor Module for Magento2 vendors are unable to add the custom attribute on their end.
But we do have an add-on Magento2 Custom Attribute Marketplace Add-on using which vendors can add a custom attribute.
Using this add-on admin can create custom attributes and select them to show on the vendor’s product creation page.
Please do contact us for any further queries or requirements. You can also check our Magento 2 Extensions for additional features.