Breckfest has work for me, unpacking a bmap file and then patching png back into a bmap file.
Major Impact, you have Breckfest.exe and the file you trying to unpack or pack back up, in the same folder?
How to use:
Drop a bmap on Breckfest.exe to get a png file.
Drop a png (dds, tga and tif also supported) on Breckfest.exe to get a bmap file.
Image to BMAP will add a ".x" to the filename (ie Skin5_C5.png would become Skin5_C5.x.bmap), this is to prevent accidentally overwriting of original files. You'll need to rename the files manually and remove the ".x"
Drop a directory on Breckfest.exe and all* supported files within that folder (not subfolders) will be processed.
Doubleclicking Breckfest.exe will process the current directory.