I went to China Unicom today. They also have an 8 RMB/month plan that comes with 30 minutes of voice and free incoming SMS when outside China. One lady said that you cannot add data to it - you will have to switch to a more expensive plan that has data and the switching is only allowed on the next billing cycle. Another lady said that you can add data. I found the employees not as friendly as China Mobile but it might be just the offices I visited.