Here is a great freeware product that does one at a time or a whole batch at once.
Developer: Martin Wright (of DXTBmp fame)
ConvImX (Extended Image Format Converter)
Extended version of ConvIm for the batch conversion of image files to alpha-aware formats. Primarily to Extended Bitmap and DDS but also to png/tiff/targa. Can be used to batch-convert DDS or Extended Bitmap from one format to another (eg 32 bit to DXT3)
4th one down from top/click yellow dot for download
http://www.mnwright.btinternet.co.uk/beta.htm
1. Select your folder with the textures from the upper-left side of ConvImX.
2. Once folder is open the textures will show in the left-lower box.
3. Select the "ALL" button to select/highlight all the textures (if you want to convert all of them at once).
4. Bottom-Center: Select what you want your images converted to from the drop down list. In your case, Extended Bitmap.
5. To convert DDS, the "Flip image data during conversion" box must be selected.
5a. (Not DDS? Convert High res to low res dxt3 by the same steps just don't flip the image)
6. Top-right: Select your output folder.
7. Bottom-Right: select your output format. Most likely DXT3.
8. Deselect Include Mipmaps, unless you like the blurries.
9. Center-bottom: click "Convert"
10. Enjoy your new textures once the conversion process is complete.
