Yes, Extra Fee extension for Magento 2 provides functionality to add additional fees while placing an order from admin back-end. You can configure whether custom fees is mandatory for admin or not. Therefore, with this extension, admin will get full flexibility.

Read this configuration guide on How to Configure Extra Fee Extension in Magento 2.

Here are the few useful resource to help.
1. How to Add Extra Fee to Magento 2 Orders and Checkout?.