Not from the back end, but you can enable the Live Search feature, and, when logged in as Admin, you can sign up individuals from the front end.
See the documentation here:
https://stephensherrardplugins.com/docs/pta-volunteer-sign-up-sheets-documentation/
Enable Sign-up form live search? — If you check this box, admin users can use the public side to sign-up other volunteers for tasks. Start typing in a first or last name on the sign-up form, and matching users/volunteers will show in a drop down list you can select to automatically fill the fields. When you submit the sign-up, that sign-up gets assigned to that user (if they have a user account on your site) instead of to your admin account.
Live Search Tables — Select which tables to search for the above live search field. You can select WordPress Users, or the Sign-Ups table, or both.