Convert a number to have 2 decimal places in Oracle

you should format the output on your display
not in the database

round(your_column, 2) as output

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see the output formatting of the display environment for options

If you are simply after output then to_char() by itself should be sufficient:

SELECT TO_CHAR(9200/100, '999.99')
FROM DUAL


+----------------------------+
| TO_CHAR(9200/100,'999.99') |
+----------------------------+
|                      92.00 |
+----------------------------+

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see it running online here:  https://rextester.com/NRWTG7393

When I divide 9200 by 100 I have 92.
If I use ROUND(92,2) I will still have 92.
What I want is the opposite. I will like to add 2 decimal places to the result. In other words xxx(92,2) should give 92.00

You are right if the number is 9234. Infact, the following statement will be correct: SELECT CAST ((9234/100) AS NUMBER(9,2)) FROM DUAL
  as the result will be 92.34
However,  SELECT CAST ((9200/100) AS NUMBER(9,2)) FROM DUAL will not give me the desired result.
The output will be 92 but I want 92.00

As has been stated previously, you need to remove the TO_NUMBER from what you are doing.

The TO_CHAR will do what you want, which is convert the number to a string and give it a format.  A NUMBER doesn't have a format.

92 = 92.0 = 92.00 = 92.000 = 92.0000

If you are using SQL*Plus, you can set the display mask for the column by doing something like this:

column display_col format 9999.00
SELECT TO_NUMBER(TO_CHAR(9200/100, '999.00')) display_col FROM DUAL;

But, as others mentioned, that is just the front end application that you are using to connect to the database formatting the number for you.  Formatting really isn't the job of the database.  If you want to force the database to do formatting, TO_CHAR is the function to use.

I finally resolved at the front end application. As advised formatting is the job of the front end application. I succeeded at this level calling a  similar  to_char and tonumber functions in SAP Business Objects

You shouldn't need a tonumber function.  It is already a number, converting a number to a number isn't useful.  You only need the tochar function, that is what does the formatting.