Samsung Electronics plans to add Apple’s iPhone 5 to existing patent lawsuits against its U.S. rival, stepping up its legal challenge to Apple as the two smartphone leaders seek to assert rights to key technologies. The fresh legal step by the South Korean group comes as Apple booked orders for over 2 million iPhone 5s in the first 24 hours of its availability. The product is due to hit stores on Friday. Samsung and Apple are locked in patent battle in 10 countries and the stakes are high as the two vie for top spot in the booming smartphone market.