Upgrading everything to Ultra R8000 except for crankset which is Shimano Sora
In short, yes but not well. 

The distance between the chainrings in a Sora crankset is different than the Ultegra crankset. So if you have an Ultegra shifter and front mech, you may experience some issues (chain rub, delayed shifting etc).

So it will work, but not as well as it should. 

An alternative is to get a 105 crankset, or other crankset of similar dimensions (like a Praxis Works for example). You could always set it all up with your Sora crankset and see how it goes. But be prepared to get another, more suitable crankset. Maybe post a different question that asks which cheaper cranksets will work with R8000.