You need customization to get this. You can read our developer documentation to get help-http://docs.wedevs.com/category/plugins/wp-project-manager/pm-dev-docs/. If you are a developer then you will be helped bye this otherwise you may require a developer to create a fields for you.
Adding a extra custom field will require customization. But there is no built in feature to add an extra field in the project. If you need this urgently then you can customize the code to get this custom field. I also notify to our developer team about this. Hope they will think about this feature.