In the Opencart Marketplace Module vendors can use the custom options which are already created by the admin. But there is no feature using which vendors can create their own custom option. If you want to provide this feature to vendors you can use our add-on Opencart Marketplace Seller custom option. Using this ad on the vendor can add the options from their end. Also, vendors can use the options created by other vendors and admin.
Please do contact us for any further queries or requirements. You can also check our Opencart Extensions for additional features.