create table testcol(col1 number,col2 number);

DECLARE
l_sqlstmt VARCHAR2(32767);
l_COLUMN_NAME VARCHAR2(32767);
BEGIN


SELECT COLUMN_NAME into l_COLUMN_NAME from user_tab_cols
where COLUMN_NAME='newcol' and table_name='testcol';

exception 
    when no_data_found then

        l_sqlstmt := 'ALTER TABLE testcol ADD(newcol DATE)';
        dbms_output.put_line(l_sqlstmt); 
        EXECUTE IMMEDIATE l_sqlstmt;

    WHEN OTHERS THEN

        DBMS_OUTPUT.PUT_LINE(SQLERRM);

end;    

SQL> desc testcol;
 Name                                      Null?    Type
 ----------------------------------------- -------- ---------------------
 COL1                                               NUMBER
 COL2                                               NUMBER

SQL> @test;
 22  /
ALTER TABLE testcol ADD(newcol DATE)

PL/SQL procedure successfully completed.

SQL> desc testcol;
 Name                                      Null?    Type
 ----------------------------------------- -------- ---------------------
 COL1                                               NUMBER
 COL2                                               NUMBER
 NEWCOL                                             DATE

SQL> @test;
 22  /
ALTER TABLE testcol ADD(newcol DATE)
DECLARE
*
ERROR at line 1:
ORA-01430: column being added already exists in table
ORA-06512: at line 15
ORA-01403: no data found

declare
associate error -0143 with an error name
begin

execute immediate 'alter table statement'

exception 
when that error then
error logging or error message
end;