Well first of all, not to discourage you but it has been done quite a few times: here look at this:http://www.omnimaga.org/index.php?topic=2283.0
So that makes it slightly useless. Ooh, and if you didn't know TI has it's owm buggy editor here: http://www.linkkabel.nl/downloads_n/ticonnect_prog_150-I-027-SA_n.exe
With that said, it's quite a challenge. Really, it's hard. About the special properties you talk about, yes there are alot of them and converting isn't easy, you can't just add .8xi to some file, it won't work.
So before you get into this rethink it on three points:
1. Personal rule: Don't reinvent the wheel (don't make something that has already been done, doesn't always stand but I am just saying..)
2. Difficulty. Once again it is quite hard.
3. Up to what point do you know the calculator?
Sorry about point 3, but you need a quite thorough understanding of the calculator, preferably also how it works in hardware. At least you need to be knowledgeable with hex and Tokens and tokenizing. These aren't things you learn and/or comprehend over-night. I think it took me a year or so before I actually was good enough to even learn Assembly (which is low-level programming) and knowing assembly is for this job pretty much a must I think, others may say otherwise but you'll be eating your heart out if you try without the proper tools.
once again, it is not my intention to discourage only to warn you: you need more then a hammer to build a house (and more then, say, a week time to build it).