Thanks for replying.
I am going to simplify my previous table in order to post my followup:

create table customers
(
cust_id int primary key,
f_name varchar2(40)
);

To generate data, I was able to do the following:

INSERT INTO customers
SELECT  cust_id.nextval,
dbms_random.string('U',trunc(dbms_random.value(1,10)))
FROM  dual
CONNECT BY level <= 10000;

I have another table which is giving me an error when I try to populate it using the code above because it is nested with my first table:

create table orders
(
orders_id int primary key,
cust_id int references customer(cust_id)
);

and when populating it:

INSERT INTO orders
SELECT orders_id.nextval,
dbms_random.string('U',trunc(dbms_random.value(1,15)))
FROM  dual
CONNECT BY level <= 12;

I got the following error:
Error report:
SQL Error: ORA-01722: invalid number
01722. 00000 - "invalid number".

What is the best way to solve this problem?
Thanks