Thanks...
HDRI maps are really simple to use... just get the hdrtoxsi.exe (https://www.finalgathering.com/tut/tut_FG/hdrtoxsi.zip ) and convert the HDRI file (eg.: https://www.debevec.org/Probes/stpeters_probe.hdr) into an Map-file.
the command for that:
hdrtoxsi your.hdri your.map
Than just copy the MAP file into your sourceimages. Now make for example an screenshader and plug an file node into color. In the file node you choose the map file (you have to allow all extensions to find it). Now use the Default Color and the Color Gain to control the brightness of your HDRI map. You should for example use an sphere to as enviroment. Disabling cast/receive shadows helps your shadow maps.
Another thing you should have in mind is, that Maya can't display the image in your viewport. A work around is, to use HDRIView to convert it into eg. an bmp file and use that for display and the map file for rendering...
If you want to render it, than just activate your MR settings for render quality (for example Production or PreviewFinalGathe) and enable Final Gather. Rays around 500 and min radius of 1 and max radius of 10 worked very well for me...
If you don't want to have the enviroment in your aplha channel, than just set the alpha color (the last one in the row, it should be white) in your screen node to black.
That's it. Using an temporary file to store the gathering solutions helps when rerendering the scene!
Hope that helps!