on nds is need asm hacking for added new slot, you need edit both OVA/ARM and table data stored outside OVA/ARM9. actually on ds is have a lot space.
for now just replace existing pokemon data, and adjust them. on DPPtHGSS is used a encrypted ncgr, shadow is need manual editing (on BWBW2 is was using a code on OVA to adjust shadow).