You can't put your own HDRI files in FPSC.... this is a game engine not a rendering application. The only thing you can do is render your own skies and use the rendered images in FPSC, unless that's what you mean.
To achieve this you would need to change all of the of the original skycube textures for one skycube found in on of the subfolder in the files\skybank folder. You need a image for the front, back, left, right, bottom and top. All of these images need to be 512x512 pixels. Just rename the original .dds files form a certain sky and replace all of these files with a .tga file with the name the renamed dds file had. the engine itself will convert the .tga files into .dds files. Make sure you don't use transparency with tga files.
Quote: "is it possible to combine speedtree with fpscreatour? "
I've got no clue at all, you probably can. If I could afford SpeedTree, I for sure wouldn't pick FPSC as my game engine, no way. I've you've got the money to afford SpeedTree, you probably can afford an engine that may work with it also.
But.... FPSC isn't created for outdoor scenes. If you're thinking of creating massive outdoor scenes with lot's of trees, then move on, don't even consider buying FPSC, it will not work. You can create outdoors scenes with FPSC, check out the game I've made by visiting my website, but you need to keep it limited. A few mountains, a few trees, rock and such and that's it. If you do too much the level will become unplayable very soon. And trust me, you reach too much way to soon with FPSC and outdoor scenes.
Besides you can create nice outdoor scenes with FPSC, but don't consider putting in characters also, your limited at 2-3 characters at the time in an outdoor level.
