Unfortunately you cannot buy train tickets at PVG.

However with plenty (more than 20) of G trains from Hongqiao to Xuzhou, your chance of getting tickets on the spot should be ok, unless you entounter peak travel during Chinese holidays. I have done that from various stations in Jiangsu to Xuzhou and vice versa and often get tickets departing within an hour.

If, however, the Shanghai - Xuzhou tickets are sold out on the G trains, what I would do is to buy a Shanghai - Nanjing ticket on a train that also stops at Xuzhou (not all the Jinghu line 京沪线 G trains do), board the train and then buy the Nanjing - Xuzhou portion from the train crew/attendant/hostess 乘务员 (This is known as 补票). The downside of course is that I probably don't have a seat for 1hr15mins. But personally I can live with that.

Note the order of stations are Shagnhai - Nanjing - Xuzhou - Beijing on the Jinghu line 京沪线.
I think right now the most sought-after present in China would be iPhone 6