if the columns aren't in identical order you can't do it, if they are and the extra columns are at the end, or beginning you can do it
it might look something like this...
create table taba (c1 number,c2 number,c3 number);
create table tabb(c4 number, c5 number);
insert into tabb values (1,1);
insert into tabb values (2,3);
insert into tabb values (3,4);
insert into tabb values (4,5);
insert into taba select tabb.*,null from tabb;