Is it possible to add a column to the [B]backend [/B]order_table via own module?
I don’t want to change existing files.
Yes, feel absolutely free to add any column to any oxtable as you like, while it’s recommened to keep on Oxid lower/upper case DB-conventions for tables/columns.
Sorry. I forgot the main thing.
I want to add a column at the backend.
For example the total sum.
No problem, you can insert your new colum(s) at any position in the table.
So you mean you want it in the Admin Backend API only.
But how can i add a column to the order table in the backend with a module.
I dont know how! 
If you are programmer, you can find a lot about modules here:
Take that as a starter with examples:
Otherwise if you like someone to do it for you, post a job.