As has been mentioned, you will need to check with the governing NRCS office and amend your CRP contract. That is fairly well a formality though. In my experience, you will need to mark up an aerial map of your place with where you want to put your plots and get some signatures, etc, but it is quite easily done. The only real stipulation I remember is to stay on your flat areas, avoiding slopes. My contract allows for 10% of the CRP acres to be devoted to food plots, I think most are like that.
From a hunting and poaching prevention standpoint though you will want to locate the plots strategically, in out of the way places as much as possible.
As far as what to plant and when... read through the old posts on this forum and you will be able to pick up a whole bunch of good info. Good luck!