October is the perfect time to pick up a cheap Halloween fog machine and fluid at many local stores, this will really help the effect you’re trying to do, especially if the fog wafts in from the light side of the door.
You can use white foam core somewhat larger than the door frame as a “white mirror” reflective source (smaller panels gaffer taped together will do), angle that about 45 degrees to the door and hit that with some 1k totas from just inside the next room/ opposite side of the set wall. Or make a wood or PVC frame or using a butterfly frame if you have them, hang an translucent white diffusion material on that and as has been said, pour on the lights from behind it.
Diffusion for this effect could be something like a plain white shower curtain from the dollar store or white gift wrapping tissue, but warning: those things are not heat-proof so be careful not to melt or ignite it with light palced too close. In this use, you want the lights farther back anyway, to maximize the coverage and eliminate hot spots. Otherwise, yeah, a Rosco or similar tough Opal frost is what I’d try first, in the frame and hit from behind.
You can do a little ramping up of the iris to help blow things out, and finish it off in post, but I’d be careful with the manual iris settings so as not to throw off theparts of the shot we want to keep, and to match the shot before the door opens.