Is it possible to say, backup a savefile from an original nds cartridge and use it on a flashcard or use a flashcard savefile and restore it on the original nds cartridge?
if you need to use SRAM (i.e. you do not have a dldi enabled cart) then with some restrictions for 2mbit saves (some work, some do not) cory1492's etool apps:
http://nds.cmamod.com/index2.htm