In the Logon History preferences you can define a number of fixed accounts in the Fixed Users section, with or without password. These fixed accounts will always be displayed and will never automatically be removed. You can additionally define submenu items for these fixed users by adding >menuname. In the example below, a submenu is created for chicago and detroit:
>chicago
scott/tiger@chicago
sys@chicago as sysdba
>detroit
scott/tiger@detroit
sys@detroit as sysdba
larry@detroit