SQL & PL/SQL :: External Table With Validation

I'm trying to create a external table column validation.

I have data like this

First column is Numeric, second is Non-Numeric

1,X
2,Y
3,Z

There are chances that I may have data as following
1,X
2A,Y
3,Z

My requirement is whenever I get non-numeric data in first column I want to pass 0 or NULL. So, I have created a external table with following script.

CREATE TABLE EXT_test
(
SL NUMBER (4),
BRANCH_NAME VARCHAR2 (100)
)
ORGANIZATION EXTERNAL
(
[code]........

Following is the code for function is numeric

function isnumeric( p_string in varchar2) return boolean as
l_number number;
begin
l_number := p_string;
return TRUE;
exception
when others then
return FALSE;
end;

It is not recognizing function IS NUMERIC. Is there any way, I can do this through External Table.

I'm getting error like

The following error has occurred:

ORA-29913: error in executing ODCIEXTTABLEOPEN callout
ORA-29400: data cartridge error
KUP-00554: error encountered while parsing access parameters
KUP-01005: syntax error: found "identifier": expecting one of: "comma, date_format, defaultif, enclosed,
(, ltrim, lrtrim, ldrtrim, notrim, nullif, optionally, ), rtrim, terminated"
KUP-01008: the bad identifier was: isnumeric
KUP-01007: at line 9 column 16
ORA-06512: at "SYS.ORACLE_LOADER", line 14
ORA-06512: at line 1