I almost never filter the camera (for diffusion anyway) these days. It’s so easy to do it in post that I don’t run the risk of doing the softening while shooting and having no option to remove the effect.
There are MANY plugs for Final Cut that will do what you need. I often use Joe’s diffuser to soften up that too hard video look. Boris FX has some nice plugs. Eve Tiffen, the filter manufacturer has gotten into the software business with a pretty complete set of plugs that emulate their glass filters. Do a Google search on the subject and you’ll find many plugs that will do what you need.
You can eveh make a do it your self version by duplicating your video layer, adding a blur or defocus filter to the top layer and then using the blend mode in FCP you can get some very nice softening effects for free,
Tom