Just discovered this post (which I had previously missed...) - Doh!

Ok so to answer your questions in a random order:

Do you accept third parties submission? - Yes, certainly do... There is no promise that they will make it into the 'official' TKL library but a couple have already. Have a look at the ones available here. Once you are done you can list yours there too regardless of whether/when it gets picked up for 'official' release.

How to do? - The details will depend on the software but generally most open source software will provide instructions for at least Ubuntu or Debian (although not always) and even if there are Ubuntu instructions that doesn't mean they will work flawlessly on Debian (the basis of TKL - v13 is built on Debian Wheezy). Personally my preferred first step is to install the software on TKL Core, or TKL LAMP or whatever TKL appliance makes the most sense as a base for the software (this step is optional if you are an experienced tinkerer - works best for me though and can avoid some headaches later if the docs miss something or a vague on any points). Carefully document everything as you go, especially any gotchas you come across...

How to build the appliance? - Once you have installed the software and got it working as you'd expect (assuming you did that) then transfer your install documentation to a TKLDev-ised GitHub repo. My advice would be to do this in a couple of stages:

1. Git/GitHub - If you're not already familiar with Git I suggest you have a bit of a play with that - although you don't need to be an expert - the basic commands will be enough... GitHub have some good 'getting started with git/GitHub' type docs.
2. Learn about TKLDev - read through the docs (and the TKLDev development docs too) and have a play with it and get familiar. Explore how the current TKL appliances are built. If you are struggling a little, I suggest modifying something that already exists to get a feel for it (that's what worked for me...)
3. Building - Once you feel ready then create your repo and start building. The TKLDev docs provide a walkthough on what goes where. This page does a full start to finish on building a new appliance that IMO will be invaluable to giving you the best shot of building something that will get picked up as an 'official' TKL appliance.

Good luck and happy developing! :) Any questions please feel free to ask. I can't offer any guarantee that I will have THE answer, but I will do my best to share what I know! :)