Hi Boris, It is possible to use Grid Layers as overlays in SSA. Check the following link for a full list of supported formats.
http://support.pb.com/help/spectrum/12.2/en/webhelp/Spatial/index.html#Spatial/source/Development/devguide/rasters_grids/raster_supportedformats.html#rasterformats
I would highly recommend using the MRR format for the best performance.