A keypad outside the protected area is always a good idea, if you make sure that its well warn on the numbers that make up your code.
Really it all depends on the risk, I am guessing you havent got anything worth protecting, otherwise you wouldnt be here you would be having a proper alarm installed. Which means the type of burglar that will target you would most likely be an opportunist and not someone that has spent time watching you come and go and planned the heist of the century. So yes putting a keypad outside the protected area will work for you, but keep it clean, and bear in mind its not recommended.