The YMDK is programmed by bootloader so you probably might want to check the documentation. Just got my COSPAD PCB yesterday which is programmed using TKG, so probably not the same as YMDK.

But for COSPAD/GHPAD via TKG it is not possible to do what you want. The num lock is a special layer within a layer. You can't change the function of 5 in this layer to anything else, nor any other number (e.g. 2 must be down arrow when num lock is off). But that doesn't stop you from doing it all in a different layer. So instead of using the num lock key, you use function keys to move between layers.