The GPS has to have its mapping data in a single "gmapsupp.img" file on the card, so you'd have to get the two data sets merged together somehow. There are (apparently) methods of using SD card data in Mapsource, so it may be possible but you'd have a fair amount of hunting around to do it.
That's quite apart from any copy protection there may be on the CN SD card.
In the end it's probably easier to get the DVD version.
Does DVD cost much more anyway?
GPSW seem to have it at the same price