I think putting a # sign before the category number will do that job.
s.menu mentions this. I am reproducing it here.
The number of categories present in the new raster map
layers will depend on the format of your site list file
(see sites.format). You can create a non-binary raster map
layer representation of your site list by creating the site
list in the format:
E|N| #n label
where E is the Easting, N is the Northing, and #n label is
the description field. The description field consists of a
pound sign # followed by the category value n to be
associated with the site's cell location, and the category
label label for n.
If the user does not include a description field starting
with
#n
beside the Easting and Northing on every line in the sites
list file, a binary raster map layer will be created
instead. In the binary raster file, each site will be
represented as the category value 1.
Hope this helps.
Sudhakar
sudhakar@ecn.purdue.edu