Try defining a custom projected coordinate system for the 'ground' data. If the ground UTM coordinates are supposed to be multipled by 0.999855 to convert them to regular, grid UTM coordinates, modify the PCS parameters like this:
false easting: 500000.0 / 0.999855 = 500072.5105
false northing: Zero so keep set to zero
scale factor: 0.9996 / 0.999855 = 0.99974496302
All other parameters remain the same. If you right-click the NAD83 UTM Zone 12N in the coordinate system picker, you should be able to choose Copy and Modify. In the new window,
Change the PCS name.
Modify the 2 projection parameters.
Save as a .prj file.
Use the Define Projection Tool or ArcCatalog to assign this definition to your ground data.
Melita