That is covered somewhere in the thread. I remember reading about it. You have to wire a relay or something. I thought about doing it myself, but decided to go with the fan always on to start with. Now I don't see a real benefit to have it stop when heat/cool stops.
Continuous circulation is probably the best, although it maybe overkill.
If you want one fan (or one group of of fans) to run for an additional x amount of time after the fridge or heater kicks off then you need to design some sensing and control circuitry to operate the fan(s). There is no simple plug in adapter that could work like that AFAIK.