You can add an external administrator to a Google Workspace by doing the following:

• Log in to the Google Admin console with a user with sufficient permissions to create and assign whatever admin role you require for the new user.
• Add the free-tier of the “Cloud Identity” service using [fly-out side menu] > Billing > Get more services, choose Cloud Identity in the left column, then Cloud Identity Free. The free “Cloud Identity” service will be added to every user. Depending on the license assignment configuration for domain.com, the Admin console may help disable automatic licenses (which will matter for the new user you are about to create, as you do not want it to receive any licenses for paid services). There is information about automatic licensing here.
•  Go to Billing > License settings – Make sure Google Workspace Automatic licensing is OFF
• Create a new user.