Long time, only the cubic team (or even only pascal?) itself was able to create such animations. The reason for this was that, as said, the animations had to be in a very special format, and the only tool that was be able to create this format were the not-so-famous WürfelTools. Included there was a tool called makedmp.exe. It took various .tga files, and made a .dat animation out of them. But noone checked how this tools worked, mainly because it had no command-line-help and the sources weren't available anymore.
But times are changing. Today, you don't have to guess how to use a utility anymore. You just have to read the documentation (yes, even you, Realtime ;). Today, you can even use a resolution of 320x200 (wow, THAT's highres).
For people who didn't guess how pascal's tool worked, I made a new one. This time with source available to all interested people. This time with .pcx images, not .tga one, because .pcx is much easier to read, and also more widely supported. This time even with some palette-aligning, so you don't have to care about same palettes anymore.