It isn't possible directly, but some people have implemented this by making an ability for each buttonpos/hotkey. For example, you could make a copy of each ability: one with the hotkey "Q", one with "W", one with "E", etc.
If you have a map with a reasonable amount of abilities per unit (1-5) and not too many abilities in total, this is pretty easy to implement. If you've triggered your spells, just register when any of those abilities are cast rather than just a single one. Similarly, anywhere you reference that particular single ability, you'll have to update it to work with any of the hotkey-copies.