How to define xkb layers for NumLock modifier?

Question

I have a custom layout based on us(intl) where I have overriden some keysyms's four layers, for example:

    key <AC04> { [          f,        F,          minus,      underscore   ] };

Now I would like to also define what should happen to these keys when NumLock is active. (I want to have the numbers on the home row when NumLock is active)

How can I take advantage of the NumLock as modifier key?

I have tried adding

    key <AC04> { [          f,        F,          minus,      underscore,  2, 2   ] };
    ...
    modifier_map NumLock { Num_Lock };

but this doesn't work.

(changes are copied to /usr/share/X11/xkb/symbols and loaded with setxkbmap -layout name)