Are you looking just to make a ticket for 1 group or department or do you have multiple IT groups that make different accounts?
I ask because we use approval processes to set up for new employees that way each step network account, phone/voicemail, Elearning account ect each go to the correct people and they perform their step the push the ticket along. This creates an audit trail as well if this is something important to your department or your HR dept it can be helpful. If your deptarment runs that whoever gets the ticket does everything that may not be necessary.
As for your direct question about how to add custom fields first go to settings --> tickets --> ticket custom fields. This is where you create the fields, the type and if they are client viewable, required ect. The interface is a bit clunky but it gets the job done.
To add a custom field to a ticket go to Setup --> Request Types --> choose a type then scroll down to custom fields then click either all or specific. If specific choose the ones you would like to use then click save. Also useful in this screen is selected departments. You can choose to only have a new employee/exiting employee ticket only show to users that are in the Human resources department for example.