Sordino reduces vibration of the bridge through the application of a mechanical implement to it - I'm sure you already know this. Interestingly, there are several different styles of mute that all impact the sound in various subtlely different ways. My unsatisfying response is that you should get whatever collection is necessary to give you sordino articulations - this will by far give you the best results.
Having said that, the usual (maybe a bit unsatisfying approach) to achieving a faux-sordino sound is to filter out a reasonable swathe of the very high frequencies (the engineers amongst this board's readers will be able to give you a much better idea which ones) and reduce the volume of the track, followed by frequent A-B'ing of the results to check you are not making it sound weird. Finally you also need to squeeze the dynamic range - you can do this by compression, but I would tend to suggest manually doing so via MIDI by just reducing the different between maximum and minimum volume, and easing off use of the velocity X-fade.
It is worth also taking a look at what you are writing and how. Most composers tend (for no particularly good reason) to stick to longer lines and phrases when writing con sordino than without - so stylistically you may wish to consider that in conjunction with the above equalisation trick.
Although a bit of an unsatisfying response, I hope it helps!