DXTBmp can read and write normalmap textures as they are still just standard DXT5 format files.
Editing bumpmaps is more of a challenge. When I looked at them when FSX first came out the default bumpmaps obviously used all parts of the texture as both the visible image and the alpha show similar "relief" patterns. DXTBmp wil let you edit the RGB and the Alpha but how those edits will combine together is complicated and I would think there must be better ways
I think the nVidia plugin for Photoshop has an option for creating normal maps. Possibly from greyscale elevation source images. Also there MUST be programs specifically designed for creating bumpmaps as it is a major part of all modern 3D modelling.
Well normalmaps are rather complicated. Each point in the "image" is actually an angle so my guess is that the information is stored using all 4 parts (RGB and A) to create a floating point value in degrees or radians.
the maths involved in converting a simple height map to a bumpmap makes my head hurt just thinking about it. You can`t just take each point and convert it. You have to analyse all the surrounding points so you know if this point is on an upslope or downslope and how steep the slope is before you can start calculating the angle of the surface normal. Lots of nasty 3D vector calculations
I think the FSX sdk recommends using the nVidia plugin/tools for creating normalmaps. Nothing I could come up with is likely to be as good as what the professionals have been doing for years