SQL & PL/SQL

hi i have a situation in my application where i have to create random username if the username which is been entered already exist in database so for example if i what to enter tJames but already exist can i suggest tJames001 tJames003 and so on something like this have anyone write pl/sql like that

something like this

create or replace function RandomString(p_Characters varchar2, p_length number)
    return varchar2
    is
      l_res varchar2(256);
    begin
      select substr(listagg(substr(p_Characters, level, 1)) within group(order by dbms_random.value), 1, p_length)
        into l_res
        from dual
      connect by level <= length(p_Characters);
     return l_res;
   end;

but it must check the username which is been try to be entered and create randon username based on that