I've done this a few times now. It works, straight from the freezer, no defrosting required.
Here are some observations, based on cooking the kernels in a stove-top metal saucepan with a little medium-hot oil over a moderate heat:
- More steam than usual comes off the frozen popcorn during cooking. Without a pretty big gap between pan and lid (bigger than usual) for all the extra steam to escape, the popcorn can come out not soggy, but soft and less-than-crisp. It's a little tricky to get the right gap without letting popcorn pop out the pan. Holding the lid flat about 2cm above the pan with one hand while moving the pan with the other seems to work.
- Even after doing this, the cooked popcorn is still a bit less than crisp immediately after cooking. If I leave it in the hot pan for about 5 minutes, with the stove off, no lid, moving it around occasionally, that seems to be enough for any extra moisture to evaporate and the popcorn to finish up nice and crisp.
- I tend to use small amounts of oil for popcorn, so spitting when I added the frozen corn wasn't a big problem. Certainly, there was less spitting than, for example, stir-frying frozen vegetables. That said, it's still worth having the lid in the other hand ready to hold over the pan immediately, but primarily to stop any fast-popping corn, more than to shield from spitting oil.
Apart from that, and maybe taking very slightly longer to pop (not nearly as big a difference as I expected, and some seem to possibly pop faster, which doesn't make much sense), it seems to cook exactly as normal straight from the freezer, no defrosting required.
So I wouldn't keep popcorn kernels in the freezer unless (like me) you need to, but if you do need to, it's no big deal, just slightly less convenient.