how to insert data from one table to another
You'd use an INSERT..SELECT query like this:

insert into custom (sku, OrderIn, OrderOut) 
select 
a.id, 
a.stock,
sum(b.prod_order)
from 
table1 a inner join table2 b on a.id=b.id
group by a.id


More details at http://dev.mysql.com/doc/refman/5.1/en/insert-select.html

The basic idea is that you build up your select query to define the set of data you want, and then use that as an input to an Insert statement to write it into the table you want to put it in.

In your question you've not defined how the created column is generated, so that isn't covered in this query.