Col legno sound channel for strings

If you find a sound font that uses it, you can do one of two things to incorporate the sound into your score. 1. Use tremolo for col legno since the tremolo channel is redundant since you have the tremolo ornaments that are sufficient. If you don't want to do that, then use an instrument change from violins to violin (to get extra mixer channels). In either case, change the sound for one of those channels to the col legno sound you find. I know they're out there, perhaps someone else will give you a link.

The best way is editing the instruments.xml file. Go to Edit -> Preferences -> Score. There is a possibility to load a second instruments list in format .xml. Check this: https://musescore.org/en/node/15803
You take the original one, search for the description of violins and in the tracks (where you already have the lines for arco, tremolo and pizzicato) copy one of these lines and rename as col legno. So you get a new track for it when you load the instrument from this list. In the "instruments.xml", give a new name to the instrument and to the instrument section in order to locate it easier when you add a new instrument or change an existing one in MuseScore. Now you only need to find a good SoundFont for col legno, unfortunately I'm still not aware of. But maybe with some sound effects on Audacity you can convert some violins' staccato or pizzicato samples into a col legno that sounds good. I've already done all this for adding (multiple) mutes to horns, trombones, tubas etc. and I think, I did even for col legno too (but not used this one yet). Check my edited file below. You may find it useful.

It is a pretty flexible way, you can add as many channels as you wish (within the limitations of the program) and even for new articulations and so on.

The answer above suggests you to change instruments. I did it before, but it is a pretty bad practice to accomplish this, because each time you go and come back from col legno, a bunch of new tracks will be added to the mixer, making it too weighty and bulky.