In principle, there is nothing wrong with this. However, freezing is a great stress for the living organisms (especially the yeasts) and sometimes the cultures do not recover and die. If you still want to try it out, you have to make sure to rinse the kefir tubers carefully with water beforehand. Then let them dry a little and put them in a suitable jar or freezer bag and freeze them.
To reactivate them after the "ice time", you should first put them in lukewarm water and only cover them with a little milk for the first preparation. We recommend that you throw away the first two preparations, as they are usually not yet really tasty again and are mainly used to feed the kefir again. The kefir definitely needs some care after freezing.