First, a proposition: It would be really cool if someone make a GUI combos editor. I'm not a programmer, but I would imagine it with a text field to edit the combo name (starting with #1) and a drop-down selector with all the available items. Actually, for each combo, there would be 2-5 selectors. (I figured out that having 6 or more items in a combo makes it crash.) The program would automatically code and save the combos and the items in the combosdb.dat.xml file. OK, enough dreaming.
After working directly with several of the Little Inferno files, I learned a lot and discovered it wasn't too hard to edit combos, letters, and even items. I'll try to organize my thoughts in this semi-tutorial for anyone interested. Be warned: you'll run into spoilers if you haven't played through the game yet.
First of all, I made an allcombos.pak file that contains the necessary icons to include in new combos the items NOT included in combos in the original game: about a dozen catalog items for purchase, plus the Sugar Plumps Portrait, the Paper Heart, the Thermometer, the Egg (Extinction), the Hug Coupon, the Delete Button, and the Mouse Pad. The download is here:
Modding the game with allcombos.pak allows to use all the items in combos without worrying about creating their icons.
Before editing anything, I suggest you make a backup copy of the file, so you can always go back to something that works. This includes the Little Inferno.exe found in the liTools folder.
To edit combos, you'll need to open vdata/combosdb.dat.xml. As suggested by Daxar, change the id of the first combo to a unique 10-digit number, and number all following combo ids consecutively. That way, the combos won't show up as already solved. (FYI, the three chimney slots have player info saved in a file in a different location, so no matter which executable you run, you'll keep the same player stats... You'll want to start a new chimney when you start modding or play a new mod.)
As you edit each combo in the combos.dat.xml, change it's name and below change the items needed to solve the combo. You'll want to copy-paste the item code from the backup combos.dat.xml (you did back it up, right?). But since some items aren't in the original game's combo list, you can also copy-paste other items from the combos.dat.xml file found in my Pop Cultures release. (To-do: a text file with all the items code for easy copy-pasting.)
As you edit the combos, you'll also remember that the first combos are from Catalog 1, the next several are from Catalog 2 (plus 1), and so on. So you won't want to use the Sun in your first combos. The combos from the unavailable catalogs show up as grayed. I don't know if the game strictly follows this break-down, but here's what I discovered:
Combos 1-6 (Bike Pirate - Movie Night) are all from Catalog 1
Combos 7-16 (Seafarers - LOL Kitty): Catalog 2 (plus 1)
Combos 17-27 (Catfish - Freaked Out Food): Catalog 3 (plus 1 and 2)
Combos 28-40 (Wooden Block - Rosy): Catalog 4
Combos 41-57 (Howl at the Moon - Pill Popper): Catalog 5
Combos 58-77 (Bearskin Run - Diseased): Catalog 6
Combos 78-99 (Chain E-mail - Future's So Bright): Catalog 7
You can require 2-5 items per combo, although only 2 or 3 show up as icons. And like I mentioned, all the items given to you by Sugar Plumps and Miss Nancy can be required to solve combos (with their proper code in the combos.dat.xml). But unless you do some editing of the letters, you can't require the Mouse Pad for a combo because your receiving it depends on solving all the combos. (However, that can be changed, which I'll explain in my next post.)
Follow Daxar's explanation to create a yourcombos.pak.filelist.txt and yourcombos.pak so you can share your combos with the rest of the world.