Nope, they save money and time by not allowing direct customizations and requiring end users to buy from resellers. However, they do typically sell multiple versions of each model with slightly different specs to appeal to those who would prefer certain customizations. Also, some Asus resellers will customize systems.
To help you start, each model number starts with a letter which designates the notebook series ( currently A, F, & N series tend to be fairly typical consumer notebooks, B series is business-oriented, K series is lower cost, G series is gaming, U and UL series are premium/high end), then a number which tells the notebook's size (20 = 12-12.9", 30 = 13-13.9", 80 = 14-14.9", 50 = 15-15.9", 60 = 16-16.9", 70 = 17-17.9", 80 = 18-18.9"), then letters that identify the type of chipset & video card (for instance, current models with a J after the number indicate Intel PM55 chipset and dedicated graphics) - also note that Asus cycles these letter descriptions every few years. After that, you'll usually find a dash and a combination of other letters and numbers, such as A1, B1, X1, etc. These indicate the customization done for the model, and can indicate different screen resolutions, hard drives, optical drives, processors, or occasionally video cards.