There's no way to "copy" per se. The only way, best way, is to open up a "create new custom food" in one tab, while having the custom food you want to "copy" open in another tab, then just entering the information into the blank fields. Sounds like a lot, but it's really not, I've done it myself.
If you haven't already, there's a bunch of other tips here:
FAQ's/Newbie Q & A, How To's, Supporting Links & Tips