将标识列值从另一个表插入到表中?

create table #test (a int identity(1,1), b varchar(20), c varchar(20))

insert into #test (b,c) values ('bvju','hjab')
insert into #test (b,c) values ('bst','sdfkg')
......
insert into #test (b,c) values ('hdsj','kfsd')
create table #test (a int identity(1,1), b varc