Hello Ilya,
Until now, I was the only one crazy enough to try and add crescendo pedals to completed organ ODFs.

Hauptwerk ver4 does have a crescendo capability that will probably satisfy most users, so I agree with Brett that you should wait for it before expending too much time on your own.
However, since you seem determined, here a few pointers. First, you need to uncompact the ODF so that you can see what you are working on. Then I think you should run my Crescendo utility on your ODF and observe all the statements that it adds to the ODF. There are a lot of them. Most of your questions will be answered by looking at such an example. You can get my Crescendo utility from my website on the "Gourmet Hauptwerk" page.
http://almorse.netBut really, unless you don't have enough to do, I don't think this effort is worth your time.