You can also use DeSmuME, an emulator for the pc which can also emulate slot-2 features by utilizing a gba save. Just import your diamond save into DeSmuME by going to file--> import backup memory. Now dump your gba save with rudolf's gba backup tool (or anything similar, but rudolf's tool can be used with a slot-1 cart). Now go to options--> gba slot. Choose gba game in the dropdown menu and select the gba pokémon game's save. Now you have to load the rom. Go to the pal park, save your game. Now go to file--> export backup memory and put it back onto your flashcart.
Do note that you most probably need to convert the save file for pokémon diamond, if this is the case then use this website: http://www.shunyweb.info