You can, you'll want to find MosesofEgypt's bitmap tool. You can extract the raw images from the .bitmap files, and then use tool to recompile them. When you recompile, be sure you're using the default DXT1 format.
Here's some examples of how much space each texture type uses based on a 2048x2048 bitmap, from MosesOfEgypt;
2048x2048 = 4,194,304 pixels
32bit = 4 bytes per pixel
16bit = 2 bytes per pixel
A8Y8 = 2 bytes per pixel
Monochrome = 1 byte per pixel
DXT1 = 8 bytes per texel
DXT3/5 = 16 bytes per texel
1 texel = 16pixels
32bit = 4,194,304 * 4 = 16,777,216 bytes(16MB)
16bit/A8Y8 = 4,194,304 * 2 = 8,388,608 bytes(8MB)
Monochrome = 4,194,304 bytes(4MB)
DXT1 = (4,194,304 / 16) * 8 = 2,097,152 bytes(2MB)
DXT3/5 = (4,194,304 / 16) * 16 = 4,194,304 bytes(4MB)
As you can see, DXT1 is 8x smaller in filesize. This is huge.