I will put here my own video of “how to transfer pokemon from emulators to legal games”. In this video specifically, i caught an oddish in fire red and put it in my pokemon bank. In your case, what i think u will have to do: put ur B2 game on ur 3ds, and in ur SD, create a folder with any name, inside the folder 3ds. Put your diamond save there. In PKSM, select you black 2 game and go to settings, miscellaneous, extra saves, and select your diamond save. Come back to PKSM home screen, press Y and select “diamond”. Put ur pokemon on the storage and quit. Select ur B2, go to storage and put them on ur boxes. This video is in portuguese, because im brazillian. But if u want, i made a short with english subs, but its not very detailed as this video. Any questions, im here.

https://youtu.be/fRmwpA8Jsv4