Builtin modal keymaps cannot be modified by addons, the user must modify them themselves through the UI. This is by design as it is not supported and is not the intended use of addon keymaps.

This is stated by developer Campbell Barton in a bug report about similar attempt.

Builtin modal keymaps cannot be modified by addons, the user must modify them themselves through the UI. This is by design since it is not supported because it is not the intended use of addon keymaps.

This is stated by developer Campbell Barton in a bug report about similar attempt.

Addons can only declare modal keymaps for its own declared modal operators.

One possible solution (which I never tested personally) is to include your own modal operator and modify its keymap instead. Depending on what it does, this operator could simply call existing operators to recreate their functionality.