How to create Resource for a user if he is not available in manage users

Summary:

Need to create Resource for a user if he is not available in manage users

Content (please ensure you mask any confidential information):

A user is trying to create a new opportunity but is not able to as that person is not available as a Resource in the 'Resource Directory' module. So we tried to create the Resource for the user but could not as he is not available as an Employee in MANAGE USERS task as well.

When we checked, we could see that the user is not present in the 'Person management' module as well. We were told that the user is not an Employee, but instead just a temporary worker.