With One Stop Order Processing (OSOP) the total weight is calculated from the Product information stored in OSOP.
If you don't have the products already defined in OSOP you should tick 'Automatically add new Products to' in the order import dialog. This will create any products used in orders that are imported (and do not already exist). Once defined you can amend the product details to include its weight. You can also import product details (including its weight) by using the Import feature in the Products Management dialog.
Once the weights are defined in OSOP any imported orders will have their total weight calculated from the OSOP defined products.
Using One Stop Order Processing with eBay, Amazon, Magento, Shopify, Etsy, Onbuy, Flubit, X-Cart, eBid, SellerDeck, Oxatis, CubeCart, osCommerce, Interspire,CRE Loaded, ekmPowershop, Pinnacle Cart, Rakuten, RomanCart, Paypal, Asos, WooCommerce , PrestaShop, Groupon, Weebly, Mail Order, Telephone Sales, CDiscount,OpenCart, Channel Advisor, Royal Mail, MyHermes, UKMail, DPD, Yodel, UPS, Metapack, Parcel 2 go, Quickbooks, Sage, Xero, Parcelforce and Stripe