You can use modifier keys in mapped keys.

Just not the ones that involve follow.
I believe IS Boxer uses \ and F11, as the defaults for FTL Follow Me and FTL Assist Me.
There's one for FTL Target Me.

You don't want those keys to be used within Warcraft.
You also don't want modifiers on those keys, within Warcraft.

When you issue the Follow command.
IS Boxer sends the F11, and adds the FTL Modifier for the slot you're in.
But then Warcraft says... he wants to initiate this keybound ability instead of follow...