osTicket intentionally differentiates between a User and an Agent and the two are separate entities and separate tables in the database. You may exist as both, but it is always true that a ticket is opened by a User and is assigned to an Agent.
If you wanted to make modifications, you could add a "New Ticket as Me" button or something and get it to do an SQL lookup on the User table using the logged in Agent email, or other identifying information. But, as KevinTheJedi said, you would be on your own as far as the customisation goes and you'd want a way to distinguish when you open a ticket if it is for yourself or on behalf of someone. A much simpler way is to just log into the User Portal as yourself and create a ticket there when needed.