This is a quick guide that am writhing on the fly. I have many questions yet, but i hope it helps.
I recomend check all the parts of one container and study well to know where are expendable parts, banners, adboards etc. Al the research i did was by test and fail.
there are a lot of things i still don't know as the ligths and more.
If someone could give us an advice about do this fine please help.
1.- Open with SW12 an rx3 container
2.- Open a "O" file in the rigth TAB
3.- Import Each part of the FIFA10 stadium to the parts on the left panel. be carefull not to erase somethintg important as goals and stuff.
4.- In the working console Adjust the bounding box with 1)view and 2) write
5.- Change your FIFA10 textures TP00,Tp01,etc to DDS
6.- Import the selected textures over the textures in the container. Do it in the correct TAB on Difusse and be carefull is not used in other partes if you don't need. The Ambient textures are yused to give shine and brigthness to some textures, but be carefull. I still don't manage this fine.
7.- The parts you don't use inside the original Rx3 container you must delete whit Insert empty vertex.
8.-Also you can use 3ds tab to insert new parts to modify or correct the stadium.
Then File and close to save your container and then import it in CM12
cheers