To transform a sites file into a raster file, use s.to.rast which by default fills one cell per site. Depending on the region settings this might be very small, so play around with the 'size=' option to make them larger.
You also have different options for interpolation from site data to raster data:
s.surf.rst - interpolation and topographic analysis from given site data to GRASS floating point raster format using regularized spline with tension
s.surf.idw - surface interpolation from sites data by Inverse Distance Squared Weighting
s.vol.rst - interpolates point data to a G3D grid volume using regularized spline with tension (RST) algorithm