Tuna can in fact be frozen but you will want to make sure that you thoroughly thaw it out first before using it as it could be dangerous not doing so. Since the last thing you want is to risk problems with food poisoning due to bacteria growth on the tuna, it will be important to properly store the tuna in the freezer. Make sure that you put the fish in an airtight sealed plastic container so that there is absolutely no risk of bacteria growing inside.
You can also use freezer bags to seal up the tuna, but the bottom line is that it is extremely important to make sure that your tuna is sealed up tight when you put in the freezer. After you have completely thawed it out after taking it out of the freezer, you can feel free to use it any way you like. There are many great recipes for tuna salad and other dishes that mix well with this type of fish.