A product in Odoo has two different forms : the product template and the product variants. Even when you're not working with variants on the products, Odoo generates a product variant for that product. In technical terms, the main product is the model (object) product.template whereas its variants is the model (object) product.product. 

Why is that important is our procedure ? Because actually, to import quotation, you need to work with product.product instead of product.template. The problem is that we can't force an ID for product product. 

What does it mean? It means that we have to create a table of correspondance and use the vlookup to actually fill in all the fields for you. 1/ You have to import the product with its ID (you can give any id you want but it has to be unique). 2/ Then you have to export again all the list of your newly created products (export the field name and the field products) => this will help you create your correspondance table3/ Use your unique identifier in a column (I used the name of the product, column B in the attached file) and do the vlookup in the column (order_line/product_id/id). 
4/ Complete all the other columns of your file and it's ready to import. 

You might encounter an "issue" when you import quotation, sometimes Odoo will not respect the sequence. For example, if your last quotation/sale order was SO001 and you import new quotations. Odoo might go directly from SO001 to SO004 with no apparent reason. Therefore, you could go to the settings > debug > technical > sequences > use 'no gap' for quotations.