The Point Mass in Mechanical Workbench creates a MASS21 element for the mechancial solver to use. If you open the ANSYS Help system, you can read the description of that element in the Element Library. The inertia inputs are Principal Inertias relative to the coordinate system used to define them.

In SpaceClaim, on the Measure tab, click the Mass Properties button then select a solid body. Assign the body a Material in the Properties window. The Principal Inertia values and axes will display. Hover over any row and you can copy the data to the clipboard. Paste that into a text editor and repeat two more times to get the three values.

I have not figured out how to get this coordinate system into Mechanical.  Looking at the options on the definition of a new coordinate system, I don’t see a way to do this. 

What I would do if I had the solid object in CAD is simply to import this into Mechanical, assign it to be a Rigid Body, create a material with the correct density for that body, and assign that material to that body. Then the correct principal inertia will be submitted when you solve the model.

Here is the code for that rigid body that was sent to the solver, it is a MASS21 element.

/com,*********** Send Body as a Rigid Body ***********
keyo,1,2,1             ! Moments of inertia in nodal coordinates
keyo,1,3,0         ! 3D Mass With Rotary Intertia

You can see the local coodinate system being defined in the code above.