I am no modder or map construction expert but in the game it is done by having a spawnpoint in the air over a bounce pad. When the grenade falls on the pad its pin will trigger.

Of course there mya be an easier way to do it but rigging spawns and pads so that grenades are dropped triggered into the room should work.

There is this block called the fun beam in the stuff section, it primes and fires anything that goes
though it, basically a bounce pad without the bounce :)