If only 25 cases are valid, that is not so bad. If you receive a combination which is not explicitly handled in your case statement, it falls into the default case and you can treat it as an error.
If you are receiving a single char from the keypad, I guess you can use the trick vaj4088 suggested but use 256 instead of 10 and use a long to store the result in. Use Serial.print to see what value each of the valid combinations of 2 digits yields and build your case statement accordingly.
If you have got any code already where you are reading form the keypad, then post it. There may be a nicer solution.