This is a great feature request. We'll leave this under review pending more votes and comments. Please share with us why requiring your donor's last name is important to your organization so we have as much context as possible for this new feature.
If you need assistance implementing custom PHP code on your website we have this guide:
Please note that this code snippet is provided as an example of how you can extend GiveWP with code. It’s up to you to implement and customize to your liking. We cannot provide support for custom code on your website, only the code that we create and distribute.