You could always batch-convert them to something else first. If you don't have a batch convertor, then do this:

Grab bmp2png/png2bmp, and save png2bmp in the same folder as the images. Opan a command prompt and change to the folder with the images. Run png2bmp *.png and wait for it to finish. Then use whatever tool you normally use to make a animated gif out of the .bmp files.