Not necessarily. The add will only work if the spatial extent is set and there are values for the NoData areas. If there are not any values there, the Add will only return the intersection of the two rasters. You would have to make the spatial extents match and then change the nodata values to 0 rather than null and then when you add them, the raster would look appropriate.
I would recommend using the Mosaic to New Raster Tool in Data Management > Raster > Raster Dataset to complete the process since the 'merge' functionality is no longer available in Raster Calculator.
This is absolutely ridiculous, taking away the flexibility and usability of raster calculator combined with the old arcInfo functions was one of the worst design flaws of Arc10...