Yes you can deep fry frozen foods but it's best to thaw the frozen foods out before you cook them for safety reasons and if the frozen food is meat you should allow it to thaw to avoid it not cooking all the way through.
When you place frozen food directly into a deep fryer of hot oil then the cold to hot contact can cause a flash back of the oil which lead to a fire.
If the food is meat and is frozen then the outside of the food would cook but the inside of the meat may still be cold and raw.
So thaw the foods out at least partially before you place the food into the deep fryer to make sure the food cooks all the way through and to prevent a flash fire from the cold frozen food coming into contact with the hot cooking oil.