Why don't you think it's feasible? Is it because it's a drum sampler and not a full blown instrument sampler? I don't really see how adding 2 extra stages to the envelopes could be difficult depending on how the program was coded. Now if adding the 2 stages to the envelopes would create too much strain on the CPU and cause sluggish performance then I can see it as not being feasible. However, I don't think adding the 2 stages would created that much of an addition strain. I could very well be wrong though.
I doubt the CPU could handle it, the 2500 doesnt have full ADSR envelopes (it doesnt right?), being the same hardware (JJ is releasing a OS for it, so it must use the same chip). If the flagship for this line of hardware (the 2500 and 1000) doesnt have it, then i doubt the possibilities of its implementation, being that it is pretty much the standard for all synths, they would have implemented it if they could. But then again... I could be very very wrong =)
forgive any typos i just got home from the lab/work and i'm wrecked.