Answer: 

As of Nov 2023, No, it is not possible to set the value of "d70" parameter in AutoPIPE Vessel when calculating a loose flange according to EN13445 App.G. The dimensions of the stub (and the chamfer on the loose flange) can be set in the UI, this should allow the value of "d70" to be set indirectly, but these values are currently ignored for EN13445 App.G. Bug 1292397 was logged and will be fixed in the next version of AutoPIPE Vessel, scheduled to be released in 2024 (baring any testing issues).