create table #t (c1 varchar(20))
insert #t values ('row 1')
insert #t values ('row 2')
insert #t values ('row 3')
select * from #t
declare @seed int,
@sql varchar(200)
set @seed = 1
set @sql = 'alter table #t add idcol int identity(' + cast(@seed as varchar) + ', 1)'
exec(@sql)
select * from #t
drop table #t