How hard it would be depends on your experience and the tools you have access to.
The first question I have to ask you is how much research have you done on this subject, please be aware that you are creating a potential bomb that could do great damage.
After saying that, yes it is possible, have you devoured the propane section here and read the threads about propane conversions, if not that would be a good place to start. Then you should be able to decide if you have the expertise to try it or maybe you would be better off to buy a smoker that is already gas fired like a GOSM (I own one) that has excellent reviews here.
The reason I'm asking you to be very cautious and well informed is I played around with a couple of different ideas of converting a charcoal smoker to propane and it's only by the grace of God I'm here to write this, I had an explosion in mine and fortunately (and miraculously) it only burned the hair on my arms.
Please be careful.