1. Using this guide backup your platinum cartridge save.
2. Get a platinum rom (google) put it on your cyclods along with your backed up save, which you have renamed to be the same file name as your rom.
3. Then using two active sessions of pokegen (one with the dimond save loaded, one with platinum save) copy all your pokemon from diamond via drag and drop.
4. Using the same utility, put your edited save back on your cartridge.
5. From there, poketransfer to black with the platinum in your ds lite and black in your 3ds
6. load pokemon from black into pokebank.
7. swap black with x.
8. using pokebank load pokemon into x.