arcgis desktop - Adding two raster datasets that overlap while maintaining extent of larger raster dataset?

I am using ArcMAP 10.0

How do I add two raster datasets that overlap while maintaining the extent of the larger raster dataset. I have thought of adding zero values to the smaller raster to then be able to add them both together in raster calculator.

Is anyone familiar with a better way to achieve this?

My data is deposit thickness data, and I want to preserve the extent of the larger raster which the smaller raster is confined within when adding their elevations together - as currently when adding the data together I just get an output raster where the data overlaps.