Hi Maxim,
What you are looking for is the Account object that goes with the current user. Retrieve the account with xpath [id = $currentUser] and this entity has the FullName attribute. Just make sure this is filled/set earlier.
Good luck!
Two options; directly retrieve the Account entity with the same id from the database, or do an inheritance split and if the outcome is an Account entity, a Cast activity to get the Account of the user.