1. Create a BoM with 2 or more lines (products) selected from available products in the database. This will be your template.
2. Select all necessary fields including all the mandatory blue fields. Create an Import Compatible Export csv file. In addition to the BOM fields, you will probably need the following fields, bom_lines/name, bom_lines/product_id/id and bom_lines/product_uom/id for each line added.
3. Modify the csv file as needed add new BOM and new lines. OpenERP will link each product_id to a product in its database.
4. Save the csv file then import back in.

Note: If the added lines (products) in BOM don't already exist in OpenERP, then you must first import all products into the products database then export them to csv format. Using the product_id in the Products csv file, create the BOMs in the BOM csv file then import them to OpenERP BOMs.