You can do this through a passive scalar transport simulation, which appears in the Simscale documentation. However, it does not appear as option in the solver selection so I do not know if it is still available.
Keep in mind that diffusion is a very very slow process at room temperature and a room is a massive length for a particle to cover by diffusion. I expect that air current induced mixing (e.g. due to the movement of people, opening/closing of a door, an open window, a computers cooling fan, convection currents from a hot object) will be the dominant process here and not diffusion. You should start with an analytical study to estimate the diffusion length scale and timescales. See the following link to calculate the diffusion time
You need to find the diffusion coefficient for your case. If we take Co2 in atmospheric air, its value is 0.0016 mm^2/s. Using the above calculator, we find that it takes 36 days for a distance of 1 meter and 326 days for 3 meters. Your particles are likely to be larger than CO2 and therefore diffusion will be even smaller.