Yes, you can tap the star nut down.
From step "F" in the owner's manual at cane Creek's web site
http://www.canecreek.com/manuals/Hea...structions.pdf
Quote:
Press the star nut into the steerer tube to a point 15 mm below the top. This should preferably be done using an installation
tool such as Cane Creek’s Star Nut Installation Tool Set (p/n TL0200). If this tool is unavailable, thread the compression bolt
into the star nut and lightly tap the assembly into position with a deadweight mallet or similar tool. Ensure that the threads of
the star nut are aligned with the steerer tube.
End quote.