(Reuters)--General Motors Corp. has tacked on an extra shift to its plant in Shanghai, raising monthly capacity by 50 percent in an effort to meet increasing car demand. GM now operates three shifts round the clock to crank out 18,000 vehicles a month, up from 12,000 when the plant ran two shifts, and plans to roll out its new Excelle sedan next month. The company is also considering assembling at least three of its prestige Cadillac models in Shanghai, initially shipping in semi-finished cars from the United States to China, where final assembly will take place.