The results you're describing definitely sound more like reticulation than true grain to me. Basically, reticulation is when the gelatin of the emulsion wrinkles up microscopically.(posted 8927 days ago)As Michael Goldfarb has suggested you can produce it deliberately by dunking the film from a hot solution into a cold one (or an alkali bath into an acid one). Results aren't very predictable though. Your best bet is to experiment with some blank film, and when you get the desired effect, use it to "sandwich" print the reticulation onto the required negative.
Modern emulsions are very tough and quite resistant to even deliberate reticulation. You might need to soften the emulsion in a 2 to 5% Sodium Hydroxide (Caustic Soda) bath first.