It has to be said: this is a very weird requirement. I would be interested to know why you are doing things this way – are you able to provide some background? But here goes ...

Building on my earlier code:

if object_id('dbo.tab1', 'U') is not null

drop table dbo.tab1;

if object_id('dbo.tab2', 'U') is not null

drop table dbo.tab2;

create table dbo.tab1

(

a int primary key

,b text

);

insert dbo.tab1

(a, b)

values (1, 'Pluto'),

(2, 'Pippo'),

(3, 'Rossi');

select *

from dbo.tab1 t;

declare @sql nvarchar(max) = '';

select @sql = concat(@sql, ',', b, ' int')

from dbo.tab1 t;

set @sql = stuff(@sql, 1, 1, ' create table dbo.tab2 (') + ')';

exec sys.sp_executesql

@sql;

--Add another row to tab1

insert dbo.tab1

(a, b)

values (4, 'Spock');

--Create the ALTER statement

set @sql = '';

select @sql = concat(@sql, ',', b, ' int')

from dbo.tab1 t

where not exists ( select 1

from sys.columns c

where c.object_id = object_id('dbo.tab2')

and c.name = cast(t.b as sysname) );

set @sql = stuff(@sql, 1, 1, 'alter table dbo.tab2 add ');

--Execute the ALTER

exec sys.sp_executesql

@sql;

select *

from dbo.tab2;