I agree with sticky, Peaches have a lot of acidity, but that is balanced with the natural sweetness. If you ferment out the peaches you'll get acidity and alcohol. There are several methods to stabilize the cider so the yeast won't kick off another round of fermentation, get that done and add the peach juice. Or, if you are going to drink the whole keg in a week or two, cold crash the cider, rack it to the keg and then add the peach juice and keep it cold. The yeast will start working, but if you keep it at beer serving temperature, and if you drink it within a couple of weeks it should be OK.