Hi,
Try the iff format. I have not had any success with the RLA because it is written differently to other apps so combustion can read the file but not access the channels.
I also just want to remind you that you need to select the IFF layer and turn on 3D depth in the Layer menu in combustion. You need to also match the Z-scale and Z-off-set.
Once this is done you will be able to composite your other elements with your #D image in combustion
Regards
Grant
Discreet UK