output geostrophic wind components

Hi,
I would like wrfout file to include the geostrophic wind components.
Is the only thing I should do is changing "i3rh" to "i03rh" in Registry.EM_COMMON
for u_g and v_g? (and then clean, configure, compile of course)

To make things clear: the question refers to a real case simulation.

No, that is not the way it works.
WRF doesn't include geostrophic wind in its output file. You need to run post-processing package (UPP or RIP) to obtain this variable. Please see the website here about these packages:
http://www2.mmm.ucar.edu/wrf/users/docs/user_guide_v4/v4.0/users_guide_chap9.html