Hello! Thank you for your question! If you need to search products by certain criteria in store manager for OpenCart, you can add here filter row: https://www.screencast.com/t/CnYyKUeFT, then put in the needed column search criteria in % https://www.screencast.com/t/n0Xd8KMAKIw, click ente and you will get the needed products in grid. Then, you can select them with Ctrl and assign to categories: https://www.screencast.com/t/tAseC6rE
Feel free to contact us also via live chat or by email (firstname.lastname@example.org) if you have any further questions! Thank you!