If, for example, the death anim took 3 seconds, and the dissipate took 3.5, then under Death Time in the object editor you add them together to make 6.5. Or add a bit extra just in case.
If you have a hero whose model has no dissipate animation, and if the death anim is 3 secs, if you set the Death Time to 3 secs, the corpse dissapears, but if you set it to 6 secs then the hero plays the death anim, then goes transparent when after for the remaining time of the Death Time (i.e. when the dissipate anim would play if it had one)
So you could make the death anim be 10 seconds if you really want it to. Just make sure that the death time is set up correctly.