hi mats ,
Thank you very much for your post.
Hope my answer will be satisfactory.
Adding suppliers in bulk is not possible yet. However, you can do this by editing the database. Searching for supplier or better the speed of searching is not just ATUM, but several factors like server speed, WooCommerce speed or the size of your database for example. We were improving performance on daily basis, but for now I have turn my devs to other tasks as developing new features. WooCommerce is due to introduce new table system that should increase the speed massively. We will wait for this to be released to see what it does to the system.
You will be in deed able to add suppliers from SC in the future, but for now this is not an option unfortunately.
Hope this helps.