[gembud] Using GEMPAK to reduce station pressure to something other than MSL

I would like to use GEMPAK to reduce station pressure (PALT) to an intermediate 
level for use in high terrain..in our case reducing it to 847mb instead of sea 
level.  We have bene doing this for many years now with a variety of inhouse 
programs, but for several reasons I would like to just do this in GEMPAK.  
However, I don't want to grid the data and then produce some sort of gridded 
file, I just want to add a parameter to a surface file that can then be 
plotted, printed, and hand analyzed.  I think it would be fairly easy to do 
with gridded data using the various math params to do the calculation..but how 
to just modify a GEMPAK surface file?

Thanks,
Robert Mullenax
CSBF Meteorology