ImageTool works in much the same way as DXTBmp. You should be able to use it for your Alpha Channels. There's no Help to speak of so you will have to figure that out yourself. I'm not sure the one from my link is the same version as the one bundled with FS. If you don't have FS2002 you can get the correct version from the FS2004 SDK download page. It should be included in one of those SDKs but I don't know which one. There should be a list of included files in the file description for each SDK.
PS. ImageTool is included in the FS2004 gmax Gamepack SDK & Terrain SDK.
http://www.microsoft.com/games/flightsimulator/fs2004_downloads_sdk.asp It's only a small file & I can mail you mine if you wish.