But I was not expecting to fight against the used file formats and authoring tools in this way.
The DevSuite that the X:R devs have used (as far I can see) does not exists any more, and this fact makes UI-Modding hard, till impossible, for us.
It seems (if I'm not wrong) that they have used "Anark Studio 4.0" to create the UI for XR.
Well, not that bad I thought!
But "Anark's Studio" was bought by "Nvidia" in the past and it seems that their new product "UI-Composer" (which is based on "Gameface") is not compatible with the structure delivered from the .CAT Files anymore.
We can read and import the .dae X:R files, (need some edititing in the dae's, texture paths etc...), but we can't export or save it in a postulated way for X:RB. Thats really, really, bad.
In detail for our Nerds:
I have imported some collada .dae into UI-Composer with success but can't generate the required bytecode.
Shure, it would be easier to buy the old Anark Gameface Suite 4.0.
But its not available anymore.
P.S.: If someone has a original Gameface Suite and want to sell, pls. send me a private msg. I'll buy it. This is a serious offer!
This are my results with UI-Composer. YeeHaa!. I can edit it, but can't save it in a proper way.

Maybe some smarter individual then me can move on from this point. I'll give up.