phenix.fake_f_obs might be of use depending on your goals. It's not a fool-proof end-user tool (in terms of convenience of use, docs, etc etc etc), and may require knowledgeable tweaking before you get a meaningful outcome; if interested talk to me off list. I used it when some one challenged me to produce Fobs that an expert would not be able to recognize as fake, and this tool did the job all right.

Pavel