I have done it before on a simple cube-like part, using a revolved cut. I made a cut section an enclosed loop that had an arc that resembled 1/4 of a circle.
My sketching plane was offset into the part by the value of the desired radius. The sketch was located so that the corner that was to be rounded, bisects the arc entity. When the sketch was complete, I specified an angle of 90
You can make a corner round by creating an advanced round. Each of the edges going into the corner should be placed in a Round Set. NOTE: Each round set can have properties similar to a Simple Round (Constant/Variable radius, Edge Chain, different Radius values, etc.). The major difference about the advanced round is that it controls the shape where all of these round sets meet (ie. the corner in this case). There are a number of different types of transitions and one of them is a corner sphere. I believe that this would give you the shape that you are looking to achieve. I think PTC's Knowledge Base has some information on how to create an Advanced Round if these instructions sound confusing. They probably have images that show you the different transitions as well. Pro/HELP may have more information on this as well.
theres an option in advanced rounds, corner sphere, thats as easy as your going to get, revolved cut will work too but you need to create an axis through the corner and its more work than you need to do