Load data from a Table that has the same structure as Table1 with the only diferrence of an ID not null column?
You could make the identity column IMPLICITLY HIDDEN.
CREATE TABLE T(I INT NOT NULL GENERATED BY DEFAULT AS IDENTITY, A INT)

ALTER TABLE T ALTER COLUMN I SET IMPLICITLY HIDDEN

INSERT INTO T VALUES (42)

SELECT I, A FROM T 

will output
I|A     
-|------
1|    42