I see that Imerge has options to add noise and grain to images, but I can't seem to find any filter to reduce noise, other than the brute force method of applying a light gaussian blur, which also removes quite a bit of detail.
It's pretty common for RAW images to require either some luma or chroma noise reduction, to the point where most RAW processing software adds some by default. Lightroom, for instance, by default sets the chroma noise reduction to 25% as soon as you enter develop mode, and Capture One by default sets both luma and chroma to 50% of maximum.
In the case where you are trying to composite two images that have different levels of noise, it would be preferable to reduce the noise in the noisier image rather than try to add noise to the cleaner one in order to make them match more closely. Of course, you could apply noise reduction first in another program and then render out the image, but then you would be losing the benefits of working directly with RAW images.