Why do u need cwm backup of stock..we already have stock rom in the form of img files which we can flash using SPFlashTool...u must be knowing this but still wanted to be clear.
N if u still want it then flash stock rom using FlashTool n select cwm recovery instead of stock recovery then back up the rom.
Anther thing u can do is,use those imgs directly...i m not sure it can be used but technically its not different then cwm's backup files..only thing u need more is nandroid.md5 file which u can create via notepad n put md5 hashes in following manner n that makes ur nandroid back up..Correct me if I m wrong.
Code:
8709d1b99e2c85229cb66bd1a87fc867 boot.img
46e542e51a4af6505f6ab84f349ff934 cache.yaffs2.img
faac12eec5aec6d17d6f8aa29aa47960 data.yaffs2.img
e25da8dd8c54824496716df8bd3bb850 recovery.img
2b7845b72a41cad9b79c60f90ef87e5f system.yaffs2.img
5df3264ede5e8a0b1cb105a0fa84fcd4 .android_secure.vfat.tar