This is actually sort of possible.
It requires placing the ID8 and a Thor into a combinator, then running the ID8 through Thor's audio inputs. The slightly tricky part is tuning the envelopes to work well together. I assigned a combinator rotary to adjust both ID8 and Thor's release envelope simultaneously so that they don't clash. It's not perfect, but it does allow for smoother long attacks than is ordinarily possible with ID8 alone. The beauty of Reason's CPU efficiency is that if I wanted to get über geeky about this I could create 12 instances of each to allow for more realistic polyphonic envelopes. Now I'm curious to try it out!