A better idea would be to open up the map pk3 file using winrar/winzip (whatever app you use) then drag the textures folder (or the contents) to the textures folder in the map pk3. Chances are there will be a textures folder in the map pack, if not just create one. That means the client will have the map and textures pak in one download.
But then *everybody* will have to download it, including those who already have the map. Does having 1 download instead of 2 downloads really make a difference?