Then it is not really a film conversion. Film has grain. You are only applying a curve
In many cases- film grain is only visible at certain enlargements- For instance, when I shot RVP50 at 5x7, printing an 8x10 or even 11x14 Ilfochrome wouldn't show any grain in the print unless it was underexposed, and you'd have to get out a magnifying glass with 645. RVP100 in 35mm had an RMS of 8- very difficult to see grain there until you get to about a 40x enlargement, so it depends on the level of emulation desired and what one is attempting to emulate (note, the OP used "emulation," not "conversion."
In any case, my sensor produces noise which is very grain-like- in fact many people equate the luminance noise from Nikon sensors to film grain in terms of distribution and overall look. So, depending on what ISO I shoot at, I can have very good film emulation by applying tonal curves to something shot at even base ISO, but if I wanted grain-like images, I'd just up the ISO a couple of stops.
Obviously though, if the issue is a loss of sharpness which isn't desired, then the only real option is to get the palette, tonality and contrast of film or film/paper combinations, not to simulate grain, which will have different characteristics depending on the film choice and the simulation of which will decrease the overall sharpness of the image.
Almost everything I shot in 35mm, 645, 6x6, 6x7, 4x5 and 5x7 was fine-grained enough that printed at 8x10 for the small negatives and 11x14 for the larger ones, even with your nose up to the print you'd be hard-pressed to see grain other than with HP5 Plus shot in 35mm and not developed in PMK, enlarged up the wazoo and under-exposed a stop or two. I certainly never had to go looking for a finer-grained film for >95% of my shooting. (My vision is 20/20 at best.)
Without going to extreme enlargements or looking under a microscope, I really couldn't see grain in prints with my staples.
RVP50 shot at EI80, RDPIII 100 shot at EI 160, Delta 100 shot at EI100 and HP5+ shot at EI400 were my staples, with the occasional 120/220 roll of E100VS or Portra 160- the positives developed in the Kodak 6-bath E-6 kit (almost always with a one stop push[1]) then enlarged onto Ilforchrome material and the B&Ws developed with PMK and generally printed on Ilford Multigrade IV. 5x7's were contact printed- everything else went on an enlarger (my enlargers were 6x7cm and 4x5" so I couldn't enlarge the 5x7 RVP50.) The Portra was all lab-developed roll and sheet. Notice that I routinely souped my positives longer in the first developer to get a higher speed and still didn't have grain visibility issues.
I did shoot some Delta 3200 at it's base (EI1000) a few times, and there you could see grain in the prints- but I'm not sure why anyone would want to emulate THAT look on a regular basis but it's trivial to do in Photoshop without any special software, but you're going to lose sharpness just like you would if you shot the film.
Paul
[1] Yes, I intentionally added extra saturation/contrast for Disneychrome- sue me.