At ArcGIS 8.1.2, you can type the commands BUILDVAT, MERGEVAT and BUILDSTA into the Raster Calculator. For example:
Input dataset is located on disk:
Inputs are in the working directory:
MERGEVAT in_grid merge_grid
Input dataset is a layer in the table of contents of ArcMap.
For more details on using the Raster Calculator, consult the ArcGIS Desktop Help: Spatial Analyst > Using the Raster Calculator > The Raster Calculator.