If it is store bought, most likely not a good idea. Most store bought contains vinegar or other acid to speed the fermentation process. My understanding that when frozen, vinegar does not provide adequate protection from microbes. Since saurkraut is not cooked there is a risk that various bacterias and pathogens could form while frozen or during the thawing process.
If it is homemade or you know that only salt is used in fermentation you should be OK. Again, this is from my fuzzy memory, but the salt would act as an adequate microbe barrier when frozen.
My dad used to make Saurkraut in 5 gallon buckets using only salt. It was usually canned in mason jars, but on occasion we would freeze a small baggie when not all of it was used. The quality wasn't quite the same, but good none the less.