In v9.3 or v10? BTW: The new v10 Map Algebra and I don't get along...If was doing this process in v93 Python it would look something like this (code not verified):#assumes that the extent of littleRaster fits entirely within extent bigRaster
bigRaster = r"C:\big"
littleRaster = r"C:\little"
gp.extent = bigRaster
#Note: no need to set the snap raster if we are just using the extent of bigRaster
somaExp = "con(isnull(" + littleRaster + "), -9999, " + littleRaster + ")" #recode NoData to -9999, otherwise keep the original values
littleRasterNew = r"C:\little_new"
gp.SingleOutputMapAlgebra_sa(somaExp, littleRasterNew)
comboRaster = r"C:\combo"
somaExp = "combine(" + bigRaster + "," + littleRasterNew + ")"
gp.SingleOutputMapAlgebra_sa(somaExp, comboRaster)