If you want modifiers...

I forget the user who did this.
But, it is a guide on the IS Boxer site.

Create a new Keymap.
And put a bunch of alternate keys in it.
For example, your normal hot keys might be: 1, 2, 3, 4, and 5.
So you would put mapped keys in the new Keymap which use the same keys to execute.
But these alternate mapped keys have a different action.

Then create a two-step (toggle) mapped key.
Set it to execute a step on press or release.
Set one of your mouse buttons as the hotkey.
Set it (Step 1) to disable your normal Keymap where 1, 2, 3, 4, 5 etc do things.
And (also Step 1) to enable your alternate Keymap, where 1-5 do different things.
Then on Step 2, do the reverse, ie, revert everything to default state.

Now, while you hold the mouse button down.
Your device which sends whatever keybind to IS Boxer..
Triggers a different action than normal, from the same keybind.
But once you release your button, everything acts normally.

Essentially, you've turned a button or key, into a modifier key.