Could i add grain to a neg by exposing a few frames of delta3200 or tri-x usig a grey card, out of focus? difrent exposures for diffrent amounts of grain, and put the photo neg and the grain neg in the enlarger, and get more grain.
Masks can be used in close contact with either the negative or the paper. If in contact with the negative, it should be used emuolsion to emulsion. A glass negative carrier may be required to maintain close enough contact. The effect might not please you as much as using grainy film for the image capture.
Probably the easiest and least technical way to add grain is to step back from your subject a healthy distance or shoot with a wide-angle lens and then raise your enlarger head and crop the way you want in the darkroom.
you can also shoot a whole roll of film on something neutral and process it for grain, then sandwich the grain with the negative you want to print. its kind of like the double exposure you suggested, but different because you can select out of your roll(s) which grain-frame looks best with your printing frame.
There is also a grain insert that fits in the back of film cameras. A friend of mine got one specifically to use for ISO 100 B/W films to add grain. The look was quite a bit more grain than even a high ISO film would give, though the effect used for the right subject can be interesting. The device looked to me like just a small piece of plastic that fit into the film channel.
When making grain masks the negative should be thinner than usual and have less contrast. Many other things such as cloth can be photographed to get interesting effects. The emulsion sides of the mask and your negative should be placed in contact in the negative carrier.