Is this supposed to be reversible?
Looks like a chunky case on the keyboard, if you cut hole in the side you could probably mount switches facing outwards. If they're too 'tall' a bigger hole could allow the case of the macropad (with the bottom removed) to rest against the PCB, though they would stick out more.
If you're really lucky depending how many you need there could be some extra key positions for ISO or JIS unused which send keycodes to wire them up to. If there are you could get low profile switches, both Kailh and Cherry released some relatively recently. This would require software on the computer to map them to do something useful.