To clarify, that folder and file won't (shouldn't) actually exist in your game's installation folder. It's a logical path that exists in Main.pak. You can get that file by unpacking Main.pak, read more here:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=504707#Post504707When unpacking, you're prompted for a destination and you DO NOT want to use the default, which is the game's own directory. Unpack it somewhere else.
The two files that seem to be relevant here are ItemCombos.txt and ItemComboPreviews.lsx. The first looks like it's a script, though with the TXT extension I'm not sure. The second is an XML file. You can use LS Tools to look at that too, though I just popped it open in Notepad++. (The XLSM appears to just be a friendly interface for generating ItemCombos.txt. That's what the macro in there does, anyway.)
The syntax in both is straightforward enough that I could figure out how to create a new recipe, but I'm not sure what to do with my modified files. I don't see anything like "Stats" in the Divinity Engine itself. I tried making the Stats folder in the game's own Public\Main folder, and tried making a Mods\Main as well, but those both throw up a "corrupt" prompt and anyway it didn't work. I imagine I need to either PAK them into a custom mod or, my own instinct, hand-editing these files won't work and there's more steps in the workflow.
Sure wish this was as straightforward as making recipes in the Skyrim CK. :\