There are several ways to do it. Per example for the CCM units_32 file I enlarged the hight of the standard Civ 3 file from 217 to 7927 pixels. So the new file had the dimensions 463 x 7927 (instead of 463 x 217). Than I added 'empty' blocs of that file (containing only the raster and the magenta colours) until the size of the enlarged file was filled up.
As you still have a broad units_32 'mega sheet', another way could be to assure that this 'mega sheet' has the pallette of your current unit_32 file and than copy your current unit_32 icons in big blocs into your new 'mega sheet'.
As I´m not a graphics specialist, may be there are much better ways to achieve these results, but as my mod CCM shows, at least these ways are working.