A Shimano 9 speed pin is 6.5mm wide, while 10 speed chains are ~6.0mm. That extra .5mm is enough to cause issues when you consider the gap between cassette cogs. Plus the ridges at one end of the chain pin won't line up.
If you had to use one out on the trail it could work, but only temporarily.
The 10 speed pins also have a different number of grooves on them. New generation 10-speed road (7901/7900/7801, 6701/6700/6600,5701/5600) and mountain (M980, HG94/74) chains use a permanent three-groove/guide connector pin