Hi carambola ,
Thank you very much for your post.
The ATUM's system to add products to POs it's the default WooCommerce's system to add products to Orders and it works in this way.
However, you can set those products as published but Private visibility. In this way, the products will be not visible in the shop (only for admins) but will be available to add it to POs and Orders. You can read more about Private visibility here.
I hope this helps,
Andres