At present it is not possible to add any fields (parameters) to a newly added or existing template in VisualEditor. This means template editing is for practical purposes currently completely broken in VE and needs a priority fix.
Reported at VE/F by user:ErgoSum: "I was trying at use VisualEditor to modify several infoboxes. I clicked "Add more information," searched for a field which returned results, and clicked on the field. The field is highlighted but does not add to the list like it should. This is the case for any fields I try to add."
ErgoSum is experiencing this in MacOS X 10.10.4 with Chrome version 44.0.2403.89, I am experiencing it in Firefox 39 and Chromium 3.0.2357.130 on XUbuntu 14.04